Add broadcast_interruption() to FrameProcessor

Replace the round-trip push_interruption_task_frame_and_wait() mechanism
with broadcast_interruption(), which pushes an InterruptionFrame both
upstream and downstream directly from the calling processor.

This eliminates race conditions (transcription arriving before the
InterruptionFrame comes back), swallowed-event timeouts (frame blocked
before reaching the sink), and the complexity of _wait_for_interruption
flag / queue bypass / frame.complete() obligations.

- Add broadcast_interruption() to FrameProcessor
- Deprecate push_interruption_task_frame_and_wait() (delegates to new method)
- Remove event field and complete() from InterruptionFrame/InterruptionTaskFrame
- Remove _wait_for_interruption flag and all special-case logic
- Remove frame.complete() calls in stt_mute_filter and llm_response_universal
- Update all 17 call sites to use broadcast_interruption()
- Update tests

src/pipecat/services/openai/stt.py

@@ -639,7 +639,7 @@ class OpenAIRealtimeSTTService(WebsocketSTTService):
         logger.debug("Server VAD: speech started")
         await self.broadcast_frame(UserStartedSpeakingFrame)
         if self._should_interrupt:
-            await self.push_interruption_task_frame_and_wait()
+            await self.broadcast_interruption()
         await self.start_processing_metrics()
 
     async def _handle_speech_stopped(self, evt: dict):