feat(neuron): OpenAI Responses API + ci cuda-check runner label

Step 2 of the Responses rollout: native `/v1/responses` endpoint on
neuron that consumes the same InferenceEvent stream as
`/v1/chat/completions` but emits it as the Responses API's named
SSE event family. No gateway-side translation.

## Surface

- `cortex-core::responses` envelope types: `ResponsesRequest`,
  `ResponsesInput` (text | items), `ResponsesInputItem` (message |
  function_call | function_call_output | reasoning),
  `ResponsesContentPart` (input_text | input_image | output_text),
  `ResponsesResponse`, `ResponsesOutputItem`, `ResponsesUsage`. Plus
  a `events::*` constant module so the projector and the wire shape
  stay in sync without string-typos.

- `neuron::wire::openai_responses`:
  - `request_to_chat(req)` flattens Responses input + instructions
    into a `ChatCompletionRequest` the candle harness already
    understands. Text-only Parts collapse to a string; mixed
    text+image Parts go to chat's content-array shape; reasoning
    items drop; function_call / function_call_output round-trip
    via tool_calls / tool_call_id metadata so the surface is
    consistent for the day the harness emits tool calls.
  - `project_responses_stream(rx, meta)` reads InferenceEvents
    and emits the eight named events that compose a Responses
    stream: response.created → output_item.added → content_part.added
    → output_text.delta×N → output_text.done → content_part.done
    → output_item.done → response.completed. Synthesises start
    frames if the producer skips Start (poisoned model, early
    disconnect) so the stream stays coherent.
  - `build_response(meta, text, reason, usage)` for the
    non-streaming path.

- `CandleHarness::inference_stream(req)` extracted from
  `chat_completion_stream`, returning a typed `InferenceStream`
  (event receiver + id/created/model_id metadata). Both
  `chat_completion_stream` and the new `responses_stream` are now
  thin wrappers that pick their wire projection. TP path got the
  same treatment (`chat_completion_tp_stream` → `inference_tp_stream`).

- `POST /v1/responses` route on neuron. Non-streaming returns one
  buffered `ResponsesResponse`; streaming returns axum SSE with
  both event names and JSON data per frame (Responses, unlike
  chat completions, uses named `event:` lines). Reused
  `inference_error_response` helper hoisted out so the chat and
  responses handlers share the InferenceError → HTTP mapping.

## CI

Also bundles the `cuda-check` runner-label fix from feedback on
commit 1859777: `runs-on: rpm` doesn't ship the CUDA toolkit so
cudarc's nvcc-version build script blew up. Switched to
`runs-on: cuda-13.0` per the existing labels.

## Scope cuts (documented in the modules)

- `previous_response_id` rejected at translate time with 400
  (`code: chained_conversation_not_supported`) — stateful chained
  conversations need a persistence layer we haven't built.
- Reasoning items dropped (no Qwen3 `<think>` routing yet).
- Single output item per response (one `"message"` carrying text);
  `function_call` items reserved but not synthesised.
- Streaming events cover the core set; `response.in_progress`
  and the web_search / image_generation event families are
  out-of-scope.

22 new tests: 5 in cortex-core (envelope round-trips), 13 in
neuron::wire (request translator + projector + non-streaming
builder), 4 in neuron's tests/api.rs (route surface — 503 when no
candle, 400 on previous_response_id, 404 on missing model for
both stream and non-stream).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/cortex-core/src/lib.rs

@@ -6,4 +6,5 @@ pub mod harness;
 pub mod metrics;
 pub mod node;
 pub mod openai;
+pub mod responses;
 pub mod translate;

crates/cortex-core/src/responses.rs

@@ -0,0 +1,341 @@
+//! OpenAI Responses API (`POST /v1/responses`) envelope types.
+//!
+//! This is OpenAI's newer chat surface, distinct from
+//! `/v1/chat/completions` in three ways that matter for us:
+//!
+//! 1. **Input shape**. Instead of a `messages` array, the request
+//!    carries `input` — either a plain string (single user turn)
+//!    or an array of typed items (messages, function calls,
+//!    function-call outputs, reasoning blocks, …).
+//! 2. **Output shape**. The response carries a single `output`
+//!    array of items, each typed. We always emit one
+//!    `OutputItem::Message` containing the assistant's reply (plus,
+//!    when we get there, separate `function_call` items).
+//! 3. **Streaming events**. Where chat completions stream
+//!    structurally-identical `chat.completion.chunk` frames over
+//!    `data:` lines, Responses streams *named* events
+//!    (`response.created`, `response.output_text.delta`,
+//!    `response.completed`, …) over `event:` + `data:` SSE pairs.
+//!    The wire projector in `neuron::wire::openai_responses` builds
+//!    these from the same [`crate::openai`]-shaped
+//!    `InferenceEvent` stream the chat projector consumes.
+//!
+//! Scope cuts for this first cut:
+//!
+//! - **`previous_response_id` is rejected at parse time**. Stateful
+//!   chained conversations need a persistence layer we don't have.
+//! - **Reasoning items are accepted-and-ignored** (no Qwen3
+//!   `<think>` routing yet). Audio and embedded resources are
+//!   rejected as unsupported.
+//! - **Tool calls** (function_call / function_call_output) are
+//!   carried as round-trip types but the candle harness doesn't
+//!   emit them yet — wired so the surface is in place for the
+//!   day we add proper tool-call extraction.
+
+use serde::{Deserialize, Serialize};
+use serde_json::Value;
+
+// ── Request ──────────────────────────────────────────────────────────
+
+/// Body of a `POST /v1/responses` request.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ResponsesRequest {
+    pub model: String,
+    pub input: ResponsesInput,
+    /// System-prompt-style instructions. The Responses API
+    /// separates these from input so a caller doesn't have to
+    /// build a `system` message item by hand.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub instructions: Option<String>,
+    #[serde(default)]
+    pub stream: bool,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub max_output_tokens: Option<u64>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub temperature: Option<f64>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub top_p: Option<f64>,
+    /// Chained-conversation identifier. We don't store responses
+    /// server-side yet; if this is `Some`, the handler returns 400.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub previous_response_id: Option<String>,
+    /// Catch-all for anything we don't model yet (tools, tool_choice,
+    /// reasoning, response_format, …). Lets a client send a
+    /// forward-compatible request without our parser rejecting it.
+    #[serde(flatten)]
+    pub extra: Value,
+}
+
+/// `input` is either a single string or an array of typed items.
+/// `#[serde(untagged)]` so the wire shape `"input": "hi"` and
+/// `"input": [{...}]` both deserialize.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(untagged)]
+pub enum ResponsesInput {
+    Text(String),
+    Items(Vec<ResponsesInputItem>),
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(tag = "type", rename_all = "snake_case")]
+pub enum ResponsesInputItem {
+    /// A user / assistant / system turn.
+    Message {
+        role: String,
+        content: ResponsesMessageContent,
+    },
+    /// Assistant emitted a tool call. Round-trip only — neuron
+    /// doesn't synthesise these yet.
+    FunctionCall {
+        call_id: String,
+        name: String,
+        arguments: String,
+    },
+    /// User is feeding a tool result back into the model.
+    FunctionCallOutput { call_id: String, output: String },
+    /// Reasoning items emitted by o-series models. Accepted but
+    /// not forwarded to the model — neuron's candle path doesn't
+    /// surface reasoning separately yet.
+    Reasoning {
+        #[serde(default)]
+        content: Vec<Value>,
+    },
+}
+
+/// Inside a `Message` item, content is either a plain string or an
+/// array of typed parts. Mirrors the chat-completions Parts shape.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(untagged)]
+pub enum ResponsesMessageContent {
+    Text(String),
+    Parts(Vec<ResponsesContentPart>),
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(tag = "type", rename_all = "snake_case")]
+pub enum ResponsesContentPart {
+    /// Plain text inside a user / system turn.
+    InputText { text: String },
+    /// An image. `image_url` is either a remote URL or a
+    /// `data:image/png;base64,…` URI; the request translator just
+    /// forwards the string.
+    InputImage {
+        image_url: String,
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        detail: Option<String>,
+    },
+    /// Returned text inside an assistant turn — only relevant when
+    /// the caller is feeding an assistant turn back in to continue
+    /// a conversation manually (no `previous_response_id`).
+    OutputText {
+        text: String,
+        #[serde(default, skip_serializing_if = "Vec::is_empty")]
+        annotations: Vec<Value>,
+    },
+}
+
+// ── Response (non-streaming) ─────────────────────────────────────────
+
+/// Body of a `POST /v1/responses` response.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ResponsesResponse {
+    pub id: String,
+    /// Always `"response"`.
+    pub object: String,
+    pub created_at: u64,
+    /// `"completed"`, `"incomplete"`, or — for the initial event of
+    /// a streaming response — `"in_progress"`.
+    pub status: String,
+    pub model: String,
+    pub output: Vec<ResponsesOutputItem>,
+    /// Populated on completion; `None` while streaming.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub usage: Option<ResponsesUsage>,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(tag = "type", rename_all = "snake_case")]
+pub enum ResponsesOutputItem {
+    Message {
+        id: String,
+        /// Always `"assistant"` for model output.
+        role: String,
+        /// Output content parts. We always emit a single
+        /// `OutputText` today; multi-part output would land here
+        /// once we have e.g. image generation.
+        content: Vec<ResponsesOutputContent>,
+        /// Item-level status. `"in_progress"` while streaming the
+        /// content parts, `"completed"` when done.
+        #[serde(default = "default_item_status")]
+        status: String,
+    },
+    /// Reserved for the day tool-call extraction lands. The wire
+    /// shape mirrors `ResponsesInputItem::FunctionCall`.
+    FunctionCall {
+        id: String,
+        call_id: String,
+        name: String,
+        arguments: String,
+        #[serde(default = "default_item_status")]
+        status: String,
+    },
+}
+
+fn default_item_status() -> String {
+    "completed".into()
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(tag = "type", rename_all = "snake_case")]
+pub enum ResponsesOutputContent {
+    OutputText {
+        text: String,
+        /// Citations / inline annotations. Empty today; reserved
+        /// for the day we wire in web search / file search.
+        #[serde(default, skip_serializing_if = "Vec::is_empty")]
+        annotations: Vec<Value>,
+    },
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ResponsesUsage {
+    pub input_tokens: u64,
+    pub output_tokens: u64,
+    pub total_tokens: u64,
+}
+
+// ── Streaming event names ────────────────────────────────────────────
+
+/// Event names the SSE projector emits, hoisted as constants so
+/// the projector and the wire shape stay in sync without
+/// string-typos. The strings are dictated by OpenAI's published
+/// Responses API.
+pub mod events {
+    pub const CREATED: &str = "response.created";
+    pub const OUTPUT_ITEM_ADDED: &str = "response.output_item.added";
+    pub const CONTENT_PART_ADDED: &str = "response.content_part.added";
+    pub const OUTPUT_TEXT_DELTA: &str = "response.output_text.delta";
+    pub const OUTPUT_TEXT_DONE: &str = "response.output_text.done";
+    pub const CONTENT_PART_DONE: &str = "response.content_part.done";
+    pub const OUTPUT_ITEM_DONE: &str = "response.output_item.done";
+    pub const COMPLETED: &str = "response.completed";
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn deserialises_input_string_form() {
+        let raw = r#"{"model": "m", "input": "hello"}"#;
+        let req: ResponsesRequest = serde_json::from_str(raw).unwrap();
+        match req.input {
+            ResponsesInput::Text(s) => assert_eq!(s, "hello"),
+            other => panic!("expected Text, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn deserialises_input_items_form() {
+        let raw = r#"{
+            "model": "m",
+            "input": [
+                {"type": "message", "role": "user", "content": "hi"}
+            ]
+        }"#;
+        let req: ResponsesRequest = serde_json::from_str(raw).unwrap();
+        match req.input {
+            ResponsesInput::Items(items) => {
+                assert_eq!(items.len(), 1);
+                match &items[0] {
+                    ResponsesInputItem::Message { role, content } => {
+                        assert_eq!(role, "user");
+                        match content {
+                            ResponsesMessageContent::Text(t) => assert_eq!(t, "hi"),
+                            other => panic!("expected Text content, got {other:?}"),
+                        }
+                    }
+                    other => panic!("expected Message item, got {other:?}"),
+                }
+            }
+            other => panic!("expected Items, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn deserialises_input_with_image() {
+        let raw = r#"{
+            "model": "m",
+            "input": [
+                {"type": "message", "role": "user", "content": [
+                    {"type": "input_text", "text": "what is this"},
+                    {"type": "input_image", "image_url": "data:image/png;base64,AAA="}
+                ]}
+            ]
+        }"#;
+        let req: ResponsesRequest = serde_json::from_str(raw).unwrap();
+        let items = match req.input {
+            ResponsesInput::Items(i) => i,
+            other => panic!("expected Items, got {other:?}"),
+        };
+        let parts = match &items[0] {
+            ResponsesInputItem::Message {
+                content: ResponsesMessageContent::Parts(p),
+                ..
+            } => p,
+            other => panic!("expected Parts, got {other:?}"),
+        };
+        assert_eq!(parts.len(), 2);
+        assert!(matches!(
+            &parts[0],
+            ResponsesContentPart::InputText { text } if text == "what is this"
+        ));
+        assert!(matches!(
+            &parts[1],
+            ResponsesContentPart::InputImage { image_url, .. }
+                if image_url == "data:image/png;base64,AAA="
+        ));
+    }
+
+    #[test]
+    fn unknown_fields_round_trip_via_extra() {
+        let raw = r#"{
+            "model": "m",
+            "input": "hi",
+            "tools": [{"type": "web_search"}],
+            "reasoning": {"effort": "medium"}
+        }"#;
+        let req: ResponsesRequest = serde_json::from_str(raw).unwrap();
+        assert!(req.extra.get("tools").is_some());
+        assert!(req.extra.get("reasoning").is_some());
+    }
+
+    #[test]
+    fn response_round_trips_through_serde() {
+        let r = ResponsesResponse {
+            id: "resp_1".into(),
+            object: "response".into(),
+            created_at: 1700,
+            status: "completed".into(),
+            model: "m".into(),
+            output: vec![ResponsesOutputItem::Message {
+                id: "msg_1".into(),
+                role: "assistant".into(),
+                content: vec![ResponsesOutputContent::OutputText {
+                    text: "hi there".into(),
+                    annotations: vec![],
+                }],
+                status: "completed".into(),
+            }],
+            usage: Some(ResponsesUsage {
+                input_tokens: 5,
+                output_tokens: 3,
+                total_tokens: 8,
+            }),
+        };
+        let json = serde_json::to_string(&r).unwrap();
+        let parsed: ResponsesResponse = serde_json::from_str(&json).unwrap();
+        assert_eq!(parsed.id, "resp_1");
+        assert_eq!(parsed.output.len(), 1);
+    }
+}