feat(core): add user transcription timeout

[chenghao-mou]

• add a new event when vad detects any speech, but stt fails to produce any transcript during that timeout

from livekit.agents import AgentSession, UserTranscriptionTimeoutEvent

  session = AgentSession(
      stt=...,
      llm=...,
      tts=...,
      # VAD heard speech but no transcript landed within 5s of the user stopping.
      # 5.0 is the default; set to None to disable.
      transcription_timeout=5.0,
  )

  @session.on("user_transcription_timeout")
  def _on_transcription_timeout(ev: UserTranscriptionTimeoutEvent) -> None:
      # ev.speech_duration   -> total VAD speech (s) this turn that produced no transcript
      # ev.vad_speech_started_at -> when the user first started speaking (epoch seconds)

      # ignore very short blips (coughs, door slams) that VAD picks up as "speech"
      if ev.speech_duration < 0.7:
          return

      session.generate_reply(
          instructions="Tell the user you didn't catch that and ask them to repeat it.",
      )

[davidzhao]

should we have a default handler with the above implementation? any downsides of having this work automatically?

[longcw]

should we have a default handler with the above implementation? any downsides of having this work automatically?

I am wondering how many false alarms it may have, it can by risky if there are some background noise.