test: add Phase 2 streaming SSE passthrough tests

Confirms the existing proxy streams SSE chunks incrementally:
- 5-chunk test with 50ms delays verifies time spread between first
  and last chunk arrival (not buffered)
- Verifies data: [DONE] terminator is forwarded

No src/ changes needed — Body::from_stream(bytes_stream()) already
handles SSE correctly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/cortex-gateway/tests/common/mod.rs

@@ -1,3 +1,8 @@
+#![allow(dead_code)]
+
+use axum::body::Body;
+use axum::http::header;
+use axum::response::Response;
 use axum::routing::{get, post};
 use axum::{Json, Router};
 use cortex_core::config::{
@@ -5,8 +10,10 @@ use cortex_core::config::{
 };
 use cortex_core::node::{ModelEntry, ModelStatus};
 use cortex_gateway::state::CortexState;
+use futures::{StreamExt, stream};
 use serde_json::{Value, json};
 use std::sync::Arc;
+use std::time::Duration;
 use tokio::net::TcpListener;
 
 /// Spawns a mock mistral.rs backend on a random port.
@@ -63,6 +70,66 @@ async fn mock_list_models() -> Json<Value> {
     }))
 }
 
+/// Spawns a mock mistral.rs backend that returns SSE streaming responses.
+/// Each chunk is delayed by `chunk_delay` to prove the proxy streams incrementally.
+pub async fn spawn_streaming_mock_backend(chunk_count: usize, chunk_delay: Duration) -> String {
+    let app = Router::new()
+        .route(
+            "/v1/chat/completions",
+            post(move |Json(body): Json<Value>| async move {
+                let model = body
+                    .get("model")
+                    .and_then(|v| v.as_str())
+                    .unwrap_or("unknown")
+                    .to_string();
+
+                let chunks: Vec<String> = (0..chunk_count)
+                    .map(|i| {
+                        let content = format!("token{i}");
+                        let chunk = json!({
+                            "id": "chatcmpl-stream-001",
+                            "object": "chat.completion.chunk",
+                            "created": 1700000000_u64,
+                            "model": model,
+                            "choices": [{
+                                "index": 0,
+                                "delta": { "content": content },
+                                "finish_reason": null
+                            }]
+                        });
+                        format!("data: {chunk}\n\n")
+                    })
+                    .collect();
+
+                let delay = chunk_delay;
+                let stream = stream::iter(
+                    chunks
+                        .into_iter()
+                        .chain(std::iter::once("data: [DONE]\n\n".to_string())),
+                )
+                .then(move |chunk| async move {
+                    tokio::time::sleep(delay).await;
+                    Ok::<_, std::convert::Infallible>(chunk)
+                });
+
+                Response::builder()
+                    .header(header::CONTENT_TYPE, "text/event-stream")
+                    .header(header::CACHE_CONTROL, "no-cache")
+                    .body(Body::from_stream(stream))
+                    .unwrap()
+            }),
+        )
+        .route("/v1/models", get(mock_list_models));
+
+    let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
+    let addr = listener.local_addr().unwrap();
+    tokio::spawn(async move {
+        axum::serve(listener, app).await.unwrap();
+    });
+
+    format!("http://{addr}")
+}
+
 /// Spawns the cortex gateway with a single node pointing at `mock_url`.
 /// The node is pre-seeded as healthy with one loaded model ("test-model").
 /// Returns the gateway's base URL.