Fix ThreadingMock call_count race condition

ThreadingMock._increment_mock_call() was not thread-safe.
Multiple threads calling the mock simultaneously could lose
increments due to race conditions on call_count and other
attributes.

Fix by overriding _increment_mock_call in ThreadingMixin
and wrapping it with the existing _mock_calls_events_lock.

Reproducer:
    m = unittest.mock.ThreadingMock()
    # Run 50 threads each calling m() 10000 times
    # Expected: 500000, Got: ~460000 (missing calls)

Author: saisneha196

Lib/unittest/mock.py

@@ -3113,6 +3113,10 @@ def _mock_call(self, *args, **kwargs):
 
         return ret_value
 
+    def _increment_mock_call(self, /, *args, **kwargs):
+        with self._mock_calls_events_lock:
+            super()._increment_mock_call(*args, **kwargs)
+
     def wait_until_called(self, *, timeout=_timeout_unset):
         """Wait until the mock object is called.