feat: add ThunderPhone realtime (speech-to-speech) plugin

[kolchinski]

Summary

Adds livekit-plugins-thunderphone: ThunderPhone voice agents as a realtime (speech-to-speech) model.

ThunderPhone's Realtime WebSocket API speaks the OpenAI Realtime protocol, so the plugin is a thin subclass of livekit-plugins-openai's RealtimeModel / RealtimeSession that adds the ThunderPhone-specific parts:

• the endpoint and its query parameters (saved agent, engine, language, caller/callee numbers, wire audio), sk_live_ keys, provider = "ThunderPhone";
• saved-agent sessions (RealtimeModel(agent_id=12)): prompt, voice, engine, languages, tools and greeting live on ThunderPhone, so the Agent's instructions/tools are not sent and the capabilities declare instructions and tools immutable;
• inline sessions (RealtimeModel(product="bolt", voice="olivia")): instructions and tools come from the LiveKit Agent; the first session.update starts the call on ThunderPhone and freezes them, so the plugin sends one fully-configured update instead of the framework's three incremental ones;
• ThunderPhone's call.* events (hang-up reason, transfer, keypad, ignored speech) as thunderphone_call_event / thunderphone_call_ended, and a clean session end when the call hangs up (no reconnect: a reconnect would start a new, separately billed call).

Docs: https://thunderphone.com/docs/guides/use-with-livekit

Installation

pip install livekit-plugins-thunderphone

Validation

• tests/test_plugin_thunderphone_realtime.py: 15 passed (URL/config shaping, held first update, call events, call-id mapping).
• Verified live against the ThunderPhone API: saved-agent and inline sessions, greeting, caller transcript, function tool round trip, barge-in, agent hang-up → call.ended → session closes cleanly, bad key → APIStatusError.
• uv lock regenerated for the new workspace member.

[CLAassistant]

All committers have signed the CLA.

[kolchinski]

Pushed fixes for the review findings and CI:

• Timeout starts another billed call: max_session_duration is now forced to None; a ThunderPhone socket is one call and must never be recycled.
• Client and server turns collide: capabilities now declare turn_detection=True and can_disable_turn_detection=False, so AgentSession never selects client-side turn handling against ThunderPhone's server-side detector.
• Option updates miss active sessions: session() registers in _sessions.
• Call-end reason bypasses log redaction: the call.ended event is logged under lk.pii.event.
• ruff and mypy are clean locally; tests: 16 passed.