SpeechTimeoutUserTurnStopStrategy previously collapsed two waits into
max(stt_timeout, user_speech_timeout), which over-waited for finalizing
STT services and could also end the turn early in a legacy code path.
Run them as independent timers instead:
- user_speech_timeout: policy floor, always runs to completion.
- stt_timeout: latency safety net, short-circuited by a finalized
transcript since STT has signaled it has nothing more to send.
The no-VAD fallback now waits only user_speech_timeout rather than
max(stt_timeout, user_speech_timeout); stt_timeout is defined relative
to VAD stop and has no meaning when no VAD event occurred. This
shortens the fallback wait for users who set stt_timeout greater than
user_speech_timeout.
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