- 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.