Fix finalization pattern: Use request/confirm in Pipecat mode, finalized flag in STT mode

- Add request_finalize() before sending ForceEndpoint in Pipecat mode
- Keep confirm_finalize() when receiving formatted finals in Pipecat mode
- Remove confirm_finalize() from STT mode (use finalized=True instead)

This follows Pipecat's two-step finalization pattern where request_finalize()
is called when sending a finalize request to the STT service, and
confirm_finalize() is called when receiving confirmation back.

src/pipecat/services/assemblyai/stt.py

@@ -438,6 +438,7 @@ class AssemblyAISTTService(WebsocketSTTService):
                 and self._websocket
                 and self._websocket.state is State.OPEN
             ):
+                await self.request_finalize()
                 await self._websocket.send(json.dumps({"type": "ForceEndpoint"}))
             await self.start_processing_metrics()
 
@@ -761,8 +762,7 @@ class AssemblyAISTTService(WebsocketSTTService):
                 self._user_speaking = True
 
             if is_final_turn:
-                if message.turn_is_formatted:
-                    self.confirm_finalize()
+                # STT mode: AssemblyAI controls finalization, just mark as finalized
                 await self.push_frame(
                     TranscriptionFrame(
                         transcript_text,