Fix MCP calls hanging after connection owner exits

[xiami762]

Summary

• wait for either the MCP command response or the connection owner task
• fail immediately with the connection root cause when the owner exits first
• cancel the pending response future when the caller is interrupted
• add regression coverage for HTTP 504 owner failures and caller cancellation

Testing

• uv run pytest -q tests/mcp tests/tool/test_delegate_task_compat.py tests/session/test_session_abort_inject.py::TestAbortPropagation::test_session_loop_run_publishes_busy_and_idle_status_events (147 passed)
• uv run ruff check flocks/mcp/client.py tests/mcp/test_mcp_client.py