Rework audio idle detection with timestamp-based adaptive sleep

Replaces the per-frame asyncio.Event signaling with a monotonic
timestamp updated on each audio frame. The handler sleeps until the
next deadline (last_audio_time + timeout), recomputing on each wake-up
to account for audio arriving during sleep.

This avoids waking the handler on every audio frame (~50/s at 20ms
chunks), and guarantees detection latency is bounded by timeout rather
than 2 * timeout.

Also renames audio_starvation_timeout to audio_idle_timeout and
associated identifiers for consistency with existing pipecat naming
(user_idle_timeout, etc.).

changelog/4244.fixed.md

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+- Fixed `VADController` getting stuck in the `SPEAKING` state when audio frames stop arriving mid-speech (e.g. user mutes mic). A new `audio_idle_timeout` parameter (default 1s, set to 0 to disable) forces a transition back to `QUIET` and emits `on_speech_stopped` when no audio is received while speaking.