feat(elevenlabs): add keyterms support to scribe v2 STT

[MuthanaA]

Summary

• Adds keyterms parameter to ElevenLabs STT for vocabulary biasing
• Keyterms help the model recognize specific words/phrases (brand names, technical terms, etc.)

Usage

  from livekit.plugins import elevenlabs                                                                                                      
                                                                                                                                              
  stt = elevenlabs.STT(                                                                                                                       
      model_id="scribe_v2",                                                                                                                   
      keyterms=["LiveKit", "WebRTC", "ElevenLabs"]                                                                                            
  )                   

Notes

• Only supported for scribe_v2 model (batch mode), not realtime
• Max 100 terms, each under 50 characters, at most 5 words per term
• https://elevenlabs.io/docs/overview/capabilities/speech-to-text#keyterm-prompting

Test plan

• Verified API accepts keyterms without errors
• uv run pytest tests/test_stt.py -k "elevenlabs" passes
• Type checking and linting pass

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[devin-ai-integration]

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