fix(tts): keep fallback child errors internal

[swayamg20]

Description

Fixes #2399 by keeping child provider errors internal while FallbackAdapter tries the next TTS. The adapter emits one terminal unrecoverable error only after every provider fails.

Changes Made

• Replace child error forwarding with private sinks, while preserving external listeners during cleanup.
• Safely handle transient StreamAdapter instances used for non-streaming providers.
• Add coverage for successful failover, terminal exhaustion, duplicate children, cleanup, and non-streaming fallback.

Pre-Review Checklist

• Build passes: @livekit/agents build, lint, typecheck, and tests pass locally
• AI-generated code reviewed: Removed unnecessary comments and ensured code quality
• Changes explained: All changes are documented and justified above
• Scope appropriate: All changes relate to the PR title
• Video demo: Not applicable to this internal TTS event-handling fix

Testing

• Automated tests added/updated
• All @livekit/agents tests pass
• restaurant_agent.ts and realtime_agent.ts: Not applicable; this is not a major change

Additional Notes

• Focused FallbackAdapter tests: 10 passed
• Full TTS suite: 100 passed
• Full Agents suite: 2,439 passed, 5 skipped

---

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

[changeset-bot]

🦋 Changeset detected

Latest commit: b48b8ff

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

This PR includes changesets to release 38 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-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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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.