fix(test): avoid false PCM failures in TTS tests

[rosetta-livekit-bot]

Summary

• Port fix(test): stop false-positive "Audio data isn't PCM" failures in TTS tests agents#6331 to the JS shared TTS test helper.
• Probe synthesized audio bytes with FFmpeg using -probesize 32 -analyzeduration 0, matching the Python test's probe-based behavior.
• Fail only when FFmpeg detects a known compressed container format, avoiding raw PCM false positives from obscure demuxer matches.

Testing

• pnpm exec prettier --check "plugins/test/src/tts.ts" "plugins/test/package.json"
• pnpm --filter @livekit/agents-plugins-test... build
• pnpm --filter @livekit/agents-plugins-test lint

[changeset-bot]

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Latest commit: 3eaf6b5

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