fix: keep VAD from cancelling a manually committed user turn

[vignesh-koruverse]

Fixes #7010.

Summary

• In manual turn detection, VAD START_OF_SPEECH no longer cancels the pending end-of-turn task. An eou task in manual mode always belongs to a turn the application already committed.
• A cancelled eou task now resets the turn-scoped flags via a done callback, unless a newer task superseded it. Stranded, _user_turn_committed makes _on_stt_event discard every later transcript until the next commit.
• Add regression tests for both behaviors and for supersession safety.

Why

commit_user_turn() resolves after scheduling the end-of-turn bounce, which sleeps the endpointing delay before delivery. Audio tail around a push-to-talk release can raise START_OF_SPEECH inside that window. The cancel is silent: no reply, no log, and the skipped cleanup leaves the session discarding all later transcripts. Same cancellation-poisoning class as #6913 and #6897.

Testing

• pytest tests/test_audio_recognition_turn_detection.py --audio_eot -q: the two behavior tests fail without the fix and pass with it
• ruff check and ruff format --check on both changed files
• mypy -p livekit.agents.voice.audio_recognition: clean

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[devin-ai-integration]

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