fix: Add missing transcription callbacks to TavusTransport for 0.0.77 compatibility

TavusTransport was broken in Pipecat 0.0.77 due to PR #2292 adding required
callbacks (on_transcription_stopped, on_transcription_error) to DailyCallbacks.

This fix adds placeholder implementations of these callbacks to TavusTransportClient,
allowing TavusTransport to initialize properly. These callbacks are not used by
Tavus (which handles avatar video, not transcription) but are required by the
DailyCallbacks validation.

Fixes initialization error:
- 2 validation errors for DailyCallbacks
- on_transcription_stopped: Field required
- on_transcription_error: Field required

src/pipecat/transports/services/tavus.py

@@ -245,6 +245,8 @@ class TavusTransportClient:
                 on_recording_started=partial(self._on_handle_callback, "on_recording_started"),
                 on_recording_stopped=partial(self._on_handle_callback, "on_recording_stopped"),
                 on_recording_error=partial(self._on_handle_callback, "on_recording_error"),
+                on_transcription_stopped=self._on_transcription_stopped,
+                on_transcription_error=self._on_transcription_error,
             )
             self._client = DailyTransportClient(
                 room_url, None, "Pipecat", self._params, daily_callbacks, self._bot_name
@@ -274,6 +276,20 @@ class TavusTransportClient:
         """Handle generic callback events."""
         logger.trace(f"[Callback] {event_name} called with args={args}, kwargs={kwargs}")
 
+    async def _on_transcription_stopped(self, stopped_by: str, stopped_by_error: bool):
+        """
+        Placeholder for transcription stopped callback.
+        Required by DailyCallbacks in Pipecat 0.0.77+ but not used by Tavus.
+        """
+        pass
+
+    async def _on_transcription_error(self, message: str):
+        """
+        Placeholder for transcription error callback.
+        Required by DailyCallbacks in Pipecat 0.0.77+ but not used by Tavus.
+        """
+        pass
+
     async def get_persona_name(self) -> str:
         """Get the persona name from the API.