fix: increase thread join timeout and clarify redundant None assignments

Changes based on Copilot AI review (2 issues):

1. Thread join timeout was shorter than cleanup timeout (Issue #1):
   - Changed _THREAD_JOIN_TIMEOUT from 5 to 12 seconds
   - Must be >= cleanup timeout (10s) plus buffer for loop.stop()
   - Prevents thread abandonment during active cleanup

2. Added detailed comment for redundant None assignments (Issue #2):
   - Explained why we set _session/_stdio_context to None even if
     _cleanup_stdio_async() already did it
   - Documents the safety cases: timeout, failure, skip, cancellation
   - Makes code intent clear for future maintainers

Author: Sourabsb

core/framework/runner/mcp_client.py

@@ -391,7 +391,7 @@ def _call_tool_http(self, tool_name: str, arguments: dict[str, Any]) -> Any:
 
     # Cleanup timeout should match or exceed connection timeout (10 seconds in _connect_stdio)
     _CLEANUP_TIMEOUT = 10  # seconds
-    _THREAD_JOIN_TIMEOUT = 5  # seconds - should be proportional to cleanup timeout
+    _THREAD_JOIN_TIMEOUT = 12  # seconds - must be >= cleanup timeout to allow full cleanup plus buffer
 
     async def _cleanup_stdio_async(self) -> None:
         """Async cleanup for STDIO session and context managers.
@@ -479,8 +479,12 @@ def disconnect(self) -> None:
                 self._loop_thread.join(timeout=self._THREAD_JOIN_TIMEOUT)
 
             # Clear remaining references
-            # Note: _session and _stdio_context are cleared in _cleanup_stdio_async() 
-            # if cleanup was successful, but we clear all references here for safety
+            # Note: _session and _stdio_context may already be None if _cleanup_stdio_async()
+            # succeeded. This redundant assignment is intentional for safety in cases where:
+            # 1. Cleanup timed out or failed
+            # 2. Cleanup was skipped (loop not running)
+            # 3. CancelledError interrupted cleanup
+            # Setting None to None is safe and ensures clean state.
             self._session = None
             self._stdio_context = None
             self._read_stream = None