fix(voice): flush zero-duration transcription segments

[kokoite]

Description

Fixes #2383.

TranscriptionSynchronizer waits for playback to start before forwarding paced text. When TTS completes with non-empty text but produces zero audio frames, no playback-started event can occur, so the segment keeps its transcript indefinitely.

This change completes a segment only when both text and audio inputs have ended and its pushed audio duration is zero. It releases the existing text-forwarding task and rotates the completed segment. Segments containing audio continue to use the existing playback-gated pacing path.

Related duplicate report: #2384.

Changes Made

• Complete and rotate zero-duration segments regardless of whether text or audio input ends first.
• Add deterministic regression coverage for zero-frame transcript delivery and the following audible segment.
• Preserve playback gating for segments that contain audio.
• Add a patch changeset for @livekit/agents.

Pre-Review Checklist

• Build passes: Build, lint, typecheck, tests, formatting, and throws checks pass locally.
• AI-generated code reviewed: Reviewed the complete base-to-head diff, naming, lifecycle races, and scope.
• Changes explained: The zero-duration state transition and preserved audio path are documented above.
• Scope appropriate: Changes are limited to the transcription synchronizer, its tests, and a changeset.
• Video demo: Not applicable; this internal edge case is reproduced and verified deterministically without external services.

Testing

• Automated tests added.
• pnpm test agents --silent — 141 files passed; 2,321 tests passed and 5 skipped.
• pnpm exec turbo run build --filter=@livekit/agents... --output-logs=errors-only — passed.
• pnpm exec turbo lint typecheck throws:check --output-logs=errors-only — 74 tasks passed.
• pnpm format:check — passed.
• Built-code reproduction delivered silent reply with zero pending rotated segments.
• restaurant_agent.ts and realtime_agent.ts: not applicable for this focused internal lifecycle fix.

Additional Notes

pnpm api:check reports existing declaration drift on current main. The same failure reproduces outside this branch, and its generated differences do not include the transcription synchronizer or any symbol changed here. This PR does not change public API.

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Note to reviewers: Please ensure the pre-review checklist is completed before starting your review.

[changeset-bot]

🦋 Changeset detected

Latest commit: 5c29f96

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 39 packages

Name	Type
@livekit/agents	Patch
@livekit/agents-plugin-anam	Patch
@livekit/agents-plugin-anthropic	Patch
@livekit/agents-plugin-assemblyai	Patch
@livekit/agents-plugin-azure	Patch
@livekit/agents-plugin-baseten	Patch
@livekit/agents-plugin-bey	Patch
@livekit/agents-plugin-cartesia	Patch
@livekit/agents-plugin-cerebras	Patch
@livekit/agents-plugin-deepgram	Patch
@livekit/agents-plugin-did	Patch
@livekit/agents-plugin-elevenlabs	Patch
@livekit/agents-plugin-fishaudio	Patch
@livekit/agents-plugin-google	Patch
@livekit/agents-plugin-hedra	Patch
@livekit/agents-plugin-hume	Patch
@livekit/agents-plugin-inworld	Patch
@livekit/agents-plugin-krisp	Patch
@livekit/agents-plugin-lemonslice	Patch
@livekit/agents-plugin-liveavatar	Patch
@livekit/agents-plugin-livekit	Patch
@livekit/agents-plugin-minimax	Patch
@livekit/agents-plugin-mistral	Patch
@livekit/agents-plugin-mistralai	Patch
@livekit/agents-plugin-neuphonic	Patch
@livekit/agents-plugin-openai	Patch
@livekit/agents-plugin-perplexity	Patch
@livekit/agents-plugin-phonic	Patch
@livekit/agents-plugin-protoface	Patch
@livekit/agents-plugin-resemble	Patch
@livekit/agents-plugin-rime	Patch
@livekit/agents-plugin-runway	Patch
@livekit/agents-plugin-sarvam	Patch
@livekit/agents-plugin-silero	Patch
@livekit/agents-plugin-soniox	Patch
@livekit/agents-plugin-tavus	Patch
@livekit/agents-plugins-test	Patch
@livekit/agents-plugin-trugen	Patch
@livekit/agents-plugin-xai	Patch

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