Replace the /sse/subscribe endpoint with a /ws WebSocket endpoint backed
by ws_mgr, and port the warmup helper (_send_sse_event -> _send_ws_event,
SSE -> Event, broadcast_sse_msg -> broadcast_msg). Also fixes two tests
left over from the prior SSE->Event/broadcast_msg rename that still
referenced the old names and were failing collection/execution.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
