Enable Soniox STT turn detection & metrics

[matejmarinko-soniox]

This PR addresses issues preventing the Soniox STT plugin from working correctly with LiveKit's turn_detection_mode="stt" and adds missing usage metrics.

Fixes #4034.

End of speech event

Previously, the plugin received <end> tokens from Soniox but did not forward them to the LiveKit agent. This caused the agent to hang or rely on timeouts to close turns. Plugin now emits SpeechEventType.END_OF_SPEECH upon receiving an <end> token from Soniox.

Usage metrics

Implemented RecognitionUsage event emission. The plugin now calculates and reports audio duration based on total_audio_proc_ms from the Soniox response.

Other changes

• Updated the default model from stt-rt-preview to stt-rt-v3. The old model is already just an alias to the stt-rt-v3 model.
• VAD refactor (Breaking change): Removed the vad parameter from init and the internal VAD task. Manual finalization using VAD events was removed. The plugin now relies entirely on Soniox's native server-side endpoint detection. This simplifies the architecture and aligns behavior with other STT plugins (e.g., Deepgram, AssemblyAI).

[chenghao-mou]

Thanks, LGTM. Tested locally with a key. This will likely be included in the release after the holidays.