Fix Whisper STT LiveKit connection options

[Duvet05]

Summary

• Update local Whisper STT adapter to accept LiveKit Agents conn_options in _recognize_impl.
• Preserve NOT_GIVEN language handling so configured STT language is still used by default.
• Add a regression test for the LiveKit STT method signature.

Why

Server logs showed the agent had STT_PROVIDER=whisper configured, but STT failed at runtime with:
WhisperSTT._recognize_impl() got an unexpected keyword argument 'conn_options'

Validation

• .venv/bin/pytest tests/test_local_providers.py tests/test_session.py tests/test_agent_entrypoint.py -q
• .venv/bin/ruff check src/local_stt.py tests/test_local_providers.py
• python3 -m py_compile src/local_stt.py tests/test_local_providers.py

Note: mypy src/local_stt.py still fails locally because faster_whisper is missing stubs/import metadata in the local env; unrelated to this signature fix.

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