services: async generators should yield frames

src/pipecat/services/lmnt.py

@@ -176,8 +176,8 @@ class LmntTTSService(TTSService):
                 await self._connect()
 
             if not self._started:
-                await self.push_frame(TTSStartedFrame())
                 await self.start_ttfb_metrics()
+                yield TTSStartedFrame()
                 self._started = True
 
             try:
@@ -186,7 +186,7 @@ class LmntTTSService(TTSService):
                 await self.start_tts_usage_metrics(text)
             except Exception as e:
                 logger.error(f"{self} error sending message: {e}")
-                await self.push_frame(TTSStoppedFrame())
+                yield TTSStoppedFrame()
                 await self._disconnect()
                 await self._connect()
                 return