[vaflag]

Voxtral 3 realtime STT can detect turns on the server. It streams turn.start, turn.eager_end, turn.resume and turn.end events. Today the plugin never sends turn_detection and drops those events, so realtime STT always needs the client-side Silero VAD.

This adds an opt-in turn_detection option that uses the same field names as the Mistral API. When it is set, the plugin sends it in a session.update right after connecting, skips Silero, and maps the events the way the Deepgram Flux plugin does:

• turn.start to START_OF_SPEECH
• turn.eager_end to PREFLIGHT_TRANSCRIPT
• turn.resume to INTERIM_TRANSCRIPT, which lets the pipeline drop a stale preflight
• turn.end to a flush, whose transcription.done becomes FINAL_TRANSCRIPT and then END_OF_SPEECH

from livekit.agents import AgentSession
from livekit.plugins import mistralai

session = AgentSession(
    stt=mistralai.STT(
        model="voxtral-mini-transcribe-realtime-3",
        turn_detection={"min_silence_ms": 300},
    ),
    turn_handling={"turn_detection": "stt"},
)

The session.update goes through the SDK connection's websocket, because mistralai<3 cannot send turn_detection yet. An explicit vad still wins. The 2602 models reject turn_detection, so they ignore it with a warning. Contradictory thresholds raise early, since the server closes the session on them.

Testing: tests/test_plugin_mistralai_stt.py runs a real SpeechStream against a fake connection (event order, a turn that starts before the previous transcript lands, validation, ignored cases). tests/test_stt.py gets a streaming entry with turn detection, which passed against the public API on voxtral-mini-transcribe-realtime-3 (WER 0.034). I added Mistral to the language exemption in that test because the realtime models return no detected language.

Not covered yet: a full AgentSession run end to end, and speech start/end times on the events. This overlaps with #7796, which touches the same _recv_task and also creates tests/test_plugin_mistralai_stt.py. Happy to rebase on whichever lands first.