fix: raise on unexpected ElevenLabs websocket close

[nightcityblade]

Summary

Raise immediately when the ElevenLabs TTS websocket closes while contexts are still pending.

Changes

• Convert unexpected websocket close events with active contexts into APIStatusError
• Preserve normal close behavior when there are no pending contexts

Testing

• python3 -m compileall -q livekit-plugins/livekit-plugins-elevenlabs/livekit/plugins/elevenlabs/tts.py
• uv run python -m compileall -q livekit-plugins/livekit-plugins-elevenlabs/livekit/plugins/elevenlabs/tts.py
• uv run ruff check livekit-plugins/livekit-plugins-elevenlabs/livekit/plugins/elevenlabs/tts.py
• uv run pytest tests/test_plugin_elevenlabs_tts.py

Fixes #5720

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[devin-ai-integration]

✅ Devin Review: No Issues Found

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

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