API: Catch Connection Cancellation on Client Disconnects

* **Gap**: Clients closing browser tabs leave active SSE streaming loops running, which wastes thread pool resources.
* **Relevant Files**:
  - `src/codegraphcontext/api/mcp_sse.py`
* **Task**:
  - Capture client cancellation flags (`RequestDisconnected`) and stop generation loops.
* **Acceptance Criteria**:
  - Closing tabs terminates worker stream loops.