Fall back to _turn_context_id in get_active_audio_context_id

TTS services whose wire protocol does not echo the context_id back on
incoming audio (Sarvam, Smallest, Soniox, Inworld, ...) call
``get_active_audio_context_id()`` to tag each chunk. That accessor
returned only ``_playing_context_id`` — the playback-side cursor set
asynchronously by ``_audio_context_task_handler`` when it pops a context
off the serialization queue.

Result: incoming audio that arrived in the gap between contexts or at
the very start of a turn (before the playback loop popped) had
``context_id=None`` and was dropped with
``unable to append audio to context: no context ID provided``.

Fall back to ``_turn_context_id`` (the synthesis-side cursor, set as
soon as the turn's context is created) so the gap is covered without
prematurely nulling the playback cursor.

src/pipecat/services/tts_service.py

@@ -1283,10 +1283,17 @@ class TTSService(AIService):
     def get_active_audio_context_id(self) -> str | None:
         """Get the active audio context ID.
 
+        Returns the playback cursor when set (during active playback), falling
+        back to the current turn's synthesis context_id. The fallback covers
+        the gap between contexts and the start of a turn before the playback
+        task has popped the just-created context off the serialization queue —
+        important for services whose wire protocol does not echo context_id
+        back on incoming audio.
+
         Returns:
-            The active context ID, or None if no context is active.
+            The active context ID, or None if neither cursor is set.
         """
-        return self._playing_context_id
+        return self._playing_context_id or self._turn_context_id
 
     async def remove_active_audio_context(self):
         """Remove the active audio context."""