feat(stt): add FakeSTT test harness for FallbackAdapter

[drain-zine]

Description

Ports Python's tests/fake_stt.py to TypeScript at agents/src/stt/testing/fake_stt.ts, exposed under stt.testing (mirrors voice.testing.FakeLLM). Follow-up to #1278 — gives STT tests the reusable harness the Python side already has.

Changes Made

• New agents/src/stt/testing/fake_stt.ts: FakeSTT, FakeRecognizeStream, FakeUserSpeech, RecognizeSentinel, speedUpFakeUserSpeech. Scripted fakeException / fakeTranscript / fakeTimeoutMs, mid-test updateOptions(), recognizeCh + streamCh observability channels, sendFakeTranscript() injection, and timed user-speech emission anchored to the first pushed frame.
• agents/src/stt/index.ts — re-exports testing subpath.
• agents/src/stt/fallback_adapter.test.ts — rewritten onto the harness; inline MockSTT/MockSpeechStream removed. One new parity test ported from Python's test_stt_recover: flips a failed instance via updateOptions() and asserts the
  recovery probe marks it available.

Pre-Review Checklist

• Build passes: All builds (lint, typecheck, tests) pass locally
• AI-generated code reviewed: Removed unnecessary comments and ensured code quality
• Changes explained: All changes are properly documented and justified above
• Scope appropriate: All changes relate to the PR title, or explanations provided for why they're included
• Video demo: N/A, internal test infrastructure only

Testing

• Automated tests added/updated (if applicable)
• All tests pass
• Make sure both restaurant_agent.ts and realtime_agent.ts - N/A, no runtime code paths touched

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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: 12cbbd9

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

This PR includes changesets to release 26 packages

Name	Type
@livekit/agents	Major
@livekit/agents-plugin-anam	Major
@livekit/agents-plugin-assemblyai	Major
@livekit/agents-plugin-baseten	Major
@livekit/agents-plugin-bey	Major
@livekit/agents-plugin-cartesia	Major
@livekit/agents-plugin-cerebras	Major
@livekit/agents-plugin-deepgram	Major
@livekit/agents-plugin-elevenlabs	Major
@livekit/agents-plugin-google	Major
@livekit/agents-plugin-hedra	Major
@livekit/agents-plugin-inworld	Major
@livekit/agents-plugin-lemonslice	Major
@livekit/agents-plugin-livekit	Major
@livekit/agents-plugin-mistral	Major
@livekit/agents-plugin-neuphonic	Major
@livekit/agents-plugin-openai	Major
@livekit/agents-plugin-phonic	Major
@livekit/agents-plugin-resemble	Major
@livekit/agents-plugin-rime	Major
@livekit/agents-plugin-runway	Major
@livekit/agents-plugin-sarvam	Major
@livekit/agents-plugin-silero	Major
@livekit/agents-plugins-test	Major
@livekit/agents-plugin-trugen	Major
@livekit/agents-plugin-xai	Major

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