fix: prevent race condition in listener iteration

Fixed RuntimeError when listeners list is modified during event emission.
The _emit_event() method now copies the listeners list before iteration
to prevent modification during async iteration.

Scenario:
1. _emit_event() iterates listeners and awaits on_event()
2. During await, cleanup or other code calls remove_listener()
3. List modified during iteration causes RuntimeError

Fix: Create a shallow copy of listeners list before iterating,
allowing safe concurrent modifications without affecting ongoing iteration.

Test results: All 23 tests in tests/realtime/test_openai_realtime.py passed

Author: gn00295120

src/agents/realtime/openai_realtime.py

@@ -266,7 +266,8 @@ def remove_listener(self, listener: RealtimeModelListener) -> None:
 
     async def _emit_event(self, event: RealtimeModelEvent) -> None:
         """Emit an event to the listeners."""
-        for listener in self._listeners:
+        # Copy list to avoid modification during iteration
+        for listener in list(self._listeners):
             await listener.on_event(event)
 
     async def _listen_for_messages(self):