Add output.audio.played message handling and update documentation

- Introduced `output.audio.played` message type for client acknowledgment of audio playback completion.
- Updated `DuplexPipeline` to track client playback state and handle playback completion events.
- Enhanced session handling to route `output.audio.played` messages to the pipeline.
- Revised API documentation to include details about the new message type and its fields.
- Updated schema documentation to reflect the addition of `output.audio.played` in the message flow.

engine/core/session.py

@@ -24,6 +24,7 @@ from models.ws_v1 import (
     SessionStopMessage,
     InputTextMessage,
     ResponseCancelMessage,
+    OutputAudioPlayedMessage,
     ToolCallResultsMessage,
 )
 
@@ -267,6 +268,14 @@ class Session:
                 logger.info(f"Session {self.id} graceful response.cancel")
             else:
                 await self.pipeline.interrupt()
+        elif isinstance(message, OutputAudioPlayedMessage):
+            await self.pipeline.handle_output_audio_played(
+                tts_id=message.tts_id,
+                response_id=message.response_id,
+                turn_id=message.turn_id,
+                played_at_ms=message.played_at_ms,
+                played_ms=message.played_ms,
+            )
         elif isinstance(message, ToolCallResultsMessage):
             await self.pipeline.handle_tool_call_results([item.model_dump() for item in message.results])
         elif isinstance(message, SessionStopMessage):