Stabilize idle-timeout coverage under CI timing

The new session_idle_timeout tests started exercising two paths the existing source annotations and timing assumptions no longer matched. CI's strict-no-cover gate now observes the session_manager return path, and the idle-reap assertion can race under slower coverage-instrumented runs. This follow-up removes the stale pragma and makes the idle-reap assertion wait for actual session removal before checking the 404 behavior.

Constraint: Must preserve the PR's runtime semantics and only address CI-validity drift
Rejected: Increase fixed sleeps again | keeps the test timing-sensitive and flaky under slower environments
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Prefer state-based waits over fixed sleeps in StreamableHTTP timeout tests
Tested: uv run --frozen pytest tests/server/test_streamable_http_manager.py; coverage run of the same suite; strict-no-cover-sensitive paths exercised locally
Not-tested: Full upstream matrix rerun after push

Author: shaun0927

src/mcp/server/lowlevel/server.py

@@ -354,7 +354,7 @@ def session_manager(self) -> StreamableHTTPSessionManager:
                 "Session manager can only be accessed after calling streamable_http_app(). "
                 "The session manager is created lazily to avoid unnecessary initialization."
             )
-        return self._session_manager  # pragma: no cover
+        return self._session_manager
 
     async def run(
         self,

tests/server/test_streamable_http_manager.py

@@ -385,8 +385,10 @@ async def mock_receive():  # pragma: no cover
 
         assert session_id is not None, "Session ID not found in response headers"
 
-        # Wait for the 50ms idle timeout to fire and cleanup to complete
-        await anyio.sleep(0.1)
+        # Wait deterministically for the idle timeout to fire and cleanup to complete.
+        with anyio.fail_after(1):
+            while session_id in manager._server_instances:
+                await anyio.sleep(0)
 
         # Verify via public API: old session ID now returns 404
         response_messages: list[Message] = []
@@ -399,6 +401,7 @@ async def capture_send(message: Message):
             "method": "POST",
             "path": "/mcp",
             "headers": [
+                (b"accept", b"application/json, text/event-stream"),
                 (b"content-type", b"application/json"),
                 (b"mcp-session-id", session_id.encode()),
             ],