fix: Move add_listener inside lock in register_flag_change_listener

Ensures both registration methods perform all tracker operations under
the lock, preventing a race where unregister could miss a listener
that hasn't been added to the tracker yet.

Co-Authored-By: mkeeler@launchdarkly.com <keelerm84@gmail.com>

Author: devin-ai-integration[bot]

contract-tests/flag_change_listener.py

@@ -36,10 +36,9 @@ def on_flag_change(flag_change: FlagChange):
             if listener_id in self._listeners:
                 self._tracker.remove_listener(self._listeners[listener_id])
 
+            self._tracker.add_listener(on_flag_change)
             self._listeners[listener_id] = on_flag_change
 
-        self._tracker.add_listener(on_flag_change)
-
     def register_flag_value_change_listener(
         self,
         listener_id: str,