fix: use pragma lax no cover for flaky exception handler coverage

This except handler is non-deterministically covered depending on
thread timing, so use lax annotation to avoid flaky CI failures.

Author: perhapzz

src/mcp/server/streamable_http.py

@@ -1015,7 +1015,7 @@ async def message_router():
                             try:
                                 # Send both the message and the event ID
                                 await self._request_streams[request_stream_id][0].send(EventMessage(message, event_id))
-                            except (anyio.BrokenResourceError, anyio.ClosedResourceError):
+                            except (anyio.BrokenResourceError, anyio.ClosedResourceError):  # pragma: lax no cover
                                 # Stream might be closed, remove from registry
                                 self._request_streams.pop(request_stream_id, None)
                         else: