Rename default LiveKit room prefix

[Duvet05]

Summary

• Rename the agent default room prefix from iot-device- to openmrs-voice-.
• Update session/event/entrypoint tests to use the clinical OpenMRS room prefix.

Validation

• .venv/bin/pytest tests/test_session.py tests/test_agent_entrypoint.py tests/test_events.py -q
• python3 -m py_compile src/agent.py src/config.py src/session.py tests/test_session.py tests/test_agent_entrypoint.py tests/test_events.py
• rg -n "iot-device" . returned no matches in the updated files

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