Widen run_stt/run_tts return type to AsyncGenerator[Frame | None, None]

The push-based STT/TTS implementations send audio/text over a socket and
receive results via a separate receive task, so there is nothing to
yield inline. They yield `None` by design. The previous declaration of
`AsyncGenerator[Frame, None]` disagreed with that, while the consumer
(`AIService.process_generator`) already accepted `Frame | None`. Widen
the producer side (abstract base and every subclass) so the type honestly
describes the contract.

Pure annotation change; no runtime behavior difference.

src/pipecat/services/gradium/stt.py

@@ -333,7 +333,7 @@ class GradiumSTTService(WebsocketSTTService):
         except Exception as e:
             logger.warning(f"Failed to send flush: {e}")
 
-    async def run_stt(self, audio: bytes) -> AsyncGenerator[Frame, None]:
+    async def run_stt(self, audio: bytes) -> AsyncGenerator[Frame | None, None]:
         """Process audio data for speech-to-text conversion.
 
         Args: