feat(helexa-acp): openai-responses provider

Stage 6a. Implements the `Provider` trait for OpenAI's Responses
API surface, parallel to the existing `OpenAIChatProvider`. Lets a
helexa-acp endpoint configured with `wire_api = "openai-responses"`
drive a `/v1/responses` server (today: neuron through cortex; later:
OpenAI directly) using the same agent-loop machinery the chat
provider already supports.

## Encoder (CompletionRequest → Responses body)

- System messages collapse into a single top-level `instructions`
  string. Multiple system messages concatenate with blank lines so
  ordering is preserved.
- User messages become `{type:"message", role:"user", content:…}`
  input items. Text content stays a bare string; MultiPart content
  (text + images, post-Stage 5) becomes a
  `[{type:"input_text"}, {type:"input_image"}]` array with images
  encoded as `data:{mime};base64,{data}` URIs — exactly the shape
  neuron's `wire::openai_responses::request_to_chat` accepts.
- Assistant text turns become an `output_text` content part inside
  a `message` item.
- Assistant tool-call turns become `function_call` input items.
- Tool result turns become `function_call_output` input items.
- `max_tokens` translates to `max_output_tokens`.

## Decoder (Responses SSE → CompletionEvent)

Reads named events on the SSE `event:` line:

- `response.output_text.delta` → `CompletionEvent::TextDelta`
- `response.output_item.added` with `type:"function_call"` →
  `CompletionEvent::ToolCallStart` (and, when the upstream
  pre-buffers fully, a single `ToolCallArgsDelta`)
- `response.function_call_arguments.delta` →
  `CompletionEvent::ToolCallArgsDelta`, correlated back to the
  tool-call slot by output_index.
- `response.completed` → `CompletionEvent::Usage` (if present) +
  `CompletionEvent::Finish` with reason mapped from `status`:
  `"completed"` → `"stop"`, `"incomplete"` → `"length"`.
- Bookkeeping events (`response.created`, `response.in_progress`,
  `*.content_part.*`, `*.output_text.done`, `*.output_item.done`,
  `*.function_call_arguments.done`, reasoning_*) are skipped.

## Wiring

- `EndpointConfig::responses_url()` joins `{base_url}/responses`.
- `WireApi::OpenAiResponses` in `build_provider` constructs the new
  provider (was previously a "reserved for future" error).
- `provider::mod.rs` registers the new module.

## Cuts (carried over from neuron-side issues)

- The decoder's `ToolCall*` handling fires correctly when the
  upstream emits `function_call` items, but the neuron candle
  harness doesn't yet (Refs #6). Real tool-call testing against
  cortex+neuron stays on the chat path until #6 lands.
- Reasoning events (`response.reasoning_*`) are deliberately
  dropped today; once neuron emits `InferenceEvent::ReasoningDelta`
  (Refs #5) the projector on the neuron side will start firing the
  reasoning event family and this decoder will need a matching
  case to route them to `CompletionEvent::ReasoningDelta`.

13 new unit tests cover encoder (system collapse, multipart user
input, assistant output_text encoding, tool-call round-trip via
function_call items) and decoder (text streaming, empty deltas
dropped, length finish, function_call lifecycle, inline-arguments
shape, cancellation, malformed payload skip).

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

crates/helexa-acp/src/config.rs

@@ -150,6 +150,11 @@ impl EndpointConfig {
         join_segments(&self.base_url, &["chat", "completions"])
     }
 
+    /// `{base_url}/responses` — OpenAI Responses API endpoint.
+    pub fn responses_url(&self) -> Url {
+        join_segments(&self.base_url, &["responses"])
+    }
+
     /// `{base_url}/models`. Called from `Provider::list_models`, which
     /// Stage 4 wires into the model-picker dropdown; until then it's
     /// reachable code with no in-tree callers.