fix: add closed state check to prevent operations on closed session

Added RuntimeError guards to send_message(), send_audio(), and interrupt()
methods to prevent operations on closed RealtimeSession instances.

Problem:
- Session has _closed flag (line 114) but only __aiter__ checks it
- send_message(), send_audio(), interrupt() methods don't verify state
- Calling these after close() leads to operations on closed model
- Can cause undefined behavior, data loss, or connection errors

Fix:
Added state check at start of each method:
- send_message(): raises "Cannot send message: session is closed"
- send_audio(): raises "Cannot send audio: session is closed"
- interrupt(): raises "Cannot interrupt: session is closed"

This provides clear, immediate feedback when attempting invalid operations
instead of silent failures or confusing downstream errors.

Test results: All 53 tests in tests/realtime/test_session.py passed

Author: gn00295120

src/agents/realtime/session.py

@@ -187,14 +187,20 @@ async def close(self) -> None:
 
     async def send_message(self, message: RealtimeUserInput) -> None:
         """Send a message to the model."""
+        if self._closed:
+            raise RuntimeError("Cannot send message: session is closed")
         await self._model.send_event(RealtimeModelSendUserInput(user_input=message))
 
     async def send_audio(self, audio: bytes, *, commit: bool = False) -> None:
         """Send a raw audio chunk to the model."""
+        if self._closed:
+            raise RuntimeError("Cannot send audio: session is closed")
         await self._model.send_event(RealtimeModelSendAudio(audio=audio, commit=commit))
 
     async def interrupt(self) -> None:
         """Interrupt the model."""
+        if self._closed:
+            raise RuntimeError("Cannot interrupt: session is closed")
         await self._model.send_event(RealtimeModelSendInterrupt())
 
     async def update_agent(self, agent: RealtimeAgent) -> None: