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/models/ws_v1.py

@@ -45,6 +45,15 @@ class ResponseCancelMessage(_StrictModel):
     graceful: bool = False
 
 
+class OutputAudioPlayedMessage(_StrictModel):
+    type: Literal["output.audio.played"]
+    tts_id: str = Field(..., min_length=1)
+    response_id: Optional[str] = None
+    turn_id: Optional[str] = None
+    played_at_ms: Optional[int] = Field(default=None, ge=0)
+    played_ms: Optional[int] = Field(default=None, ge=0)
+
+
 class ToolCallResultStatus(_StrictModel):
     code: int
     message: str
@@ -67,6 +76,7 @@ CLIENT_MESSAGE_TYPES = {
     "session.stop": SessionStopMessage,
     "input.text": InputTextMessage,
     "response.cancel": ResponseCancelMessage,
+    "output.audio.played": OutputAudioPlayedMessage,
     "tool_call.results": ToolCallResultsMessage,
 }