fix(soniox): send per-stream keepalive to prevent 408 timeout mid-utterance

[tsushanth]

Fixes #6225

Root cause

In agents 1.6.x the framework drives a single long-lived SynthesizeStream per turn and feeds text incrementally as LLM tokens arrive. Between token chunks — and especially during tool calls — the stream can sit idle for many seconds. Soniox enforces a per-stream idle timeout and responds with error code 408 when it fires. That exception propagates up through _main_task and tears down the agent's TTS turn entirely.

The existing connection-level keepalive ({"keep_alive": true} without a stream_id, sent every 10 s) keeps the WebSocket alive between streams but does not reset the per-stream idle timer on the Soniox server — the server tracks each stream independently.

Fix

Add a _KeepAliveStream outbound message type that carries a stream_id. The _keepalive_loop now runs on a 5 s tick and, every 15 s, enqueues {"stream_id": "...", "keep_alive": true} for every active stream that has already sent its config but has not yet seen audio_end. The message is routed through the same _input_queue the send loop already serialises, so there is no new locking surface.

15 s is well within Soniox's ~30 s per-stream idle limit, so the timeout no longer fires during normal inter-token pauses or tool-call gaps.

Changes

• livekit-plugins/livekit-plugins-soniox/livekit/plugins/soniox/tts.py
  • Add _STREAM_KEEPALIVE_INTERVAL = 15 constant with explanatory comment
  • Add _KeepAliveStream dataclass and include it in _OutboundMsg
  • Handle _KeepAliveStream in _send_loop by emitting {"stream_id": ..., "keep_alive": true}
  • Refactor _keepalive_loop to tick every 5 s; fire the connection-level keepalive on the existing 10 s cadence and the per-stream keepalive on the 15 s cadence

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[davidzhao]

@tsushanth could you run make fix to get CI back to passing?