fix(mcp): fix lifecycle-task leak in stateful clients and refactor shared logic…

[qbc2016]

Problem

MCP subprocesses were accumulating as children of the qwenpaw app daemon across agent hot-reloads (#4105). Root cause: StdIOStatefulClient.close() and HttpStatefulClient.close() returned early when is_connected=False, even if the background lifecycle task was still running in a reconnect loop. Every hot-reload called mcp_manager.close_all(), but the reconnecting clients were silently skipped — their tasks eventually spawned fresh subprocesses that were never cleaned up.

A secondary issue (#4100): HttpStatefulClient did not recover from transport-layer failures (anyio.ClosedResourceError, HTTP timeouts, etc.) because the error never propagated back to the lifecycle loop.

Changes

Bug fixes

• close() lifecycle-task leak — always stop the lifecycle task when it is running, regardless of is_connected state (is_connected=False + running task = reconnect loop, not a clean idle state).
• close() stale _lifecycle_task reference on cancellation — moved self._lifecycle_task = None to a finally block so it is cleared even when the calling coroutine is cancelled (CancelledError is BaseException, not Exception).
• connect() spurious ready on reconnect — cleared _ready_event before creating the lifecycle task so a second connect() after close() waits for the new connection rather than returning immediately.
• reload() returning before reconnect completes — cleared _ready_event before waiting so the caller blocks until the new session is actually established.
• connect() double lifecycle-task on transport error — added has_task guard to prevent creating a second task when one is already running in a reconnect loop.
• Transport-error auto-recovery (#4100) — introduced _handle_transport_error() and _TRANSPORT_ERRORS to detect broken-pipe, EOF, and anyio / httpx transport errors in call_tool / list_tools and trigger a reconnect via _reload_event, fixing the case where streamable_http_client's internal post_writer task silently closes the
  write stream.
• _force_cleanup_client silent no-op (manager.py) — replaced the dead client.stack attribute access (which was always None for the new clients) with a direct client.close(ignore_errors=True) call.
• replace_client() holding lock during close() (manager.py) — moved old_client.close() outside the lock to match remove_client()'s pattern and avoid blocking get_clients() / close_all() for the duration of the lifecycle-task shutdown.

Refactoring

• Extracted _MCPClientMixin containing all shared logic (_run_lifecycle, connect, reload, close, list_tools, call_tool, _handle_transport_error, _validate_connection). Each concrete class now only implements __init__ and _setup_transport.
• reload() was previously only on StdIOStatefulClient; it is now available on HttpStatefulClient as well via the mixin.
• Consolidated all transport-error exception types into a single _TRANSPORT_ERRORS tuple for clarity.

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