fix(llm): preserve provider events on restart

[Bortlesboat]

Summary

• forward provider-specific realtime session events through the fallback wrapper
• rebind those event forwarders whenever the underlying realtime session is restarted
• keep subscriptions made during a restart detached from the closing child session

Root cause

The fallback session only forwarded its fixed core event set. Provider-specific events such as openai_server_event_received were available only on the private active child, and that child is replaced by restart_session().

Applications can now subscribe to provider-specific events on the stable fallback session instead:

session.on("openai_server_event_received", handler)

Testing

• pytest tests/test_realtime_fallback.py -q (38 passed)
• ruff check livekit-agents/livekit/agents/llm/realtime_fallback_adapter.py tests/test_realtime_fallback.py
• ruff format --check livekit-agents/livekit/agents/llm/realtime_fallback_adapter.py tests/test_realtime_fallback.py
• mypy --config-file pyproject.toml --untyped-calls-exclude=smithy_aws_core -p livekit.agents.llm.realtime_fallback_adapter

Fixes #6559

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[devin-ai-integration]

✅ Devin Review: No Issues Found

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