[3.15] gh-145235: Make dict watcher API thread-safe for free-threaded builds (gh...

[3.15] gh-145235: Make dict watcher API thread-safe for free-threaded builds (gh-145233) (#149690)

In free-threaded builds, concurrent calls to PyDict_AddWatcher, PyDict_ClearWatcher, PyDict_Watch, and PyDict_Unwatch can race on the shared callback array and the per-dict watcher tags. This change adds a mutex to serialize watcher registration and removal, atomic operations for tag updates, and atomic acquire/release synchronization for callback dispatch in _PyDict_SendEvent.

(cherry picked from commit 8a4895985f42282504d83b9bd0c77b129f95a5d5)

Co-authored-by: Alper <alperyoney@fb.com>