Emit AgentConfigUpdate in OTLP session logs

[theomonnom]

No description provided.

[devin-ai-integration]

✅ Devin Review: No Issues Found

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

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[toubatbrian]

This is an automated Claude Code Routine created by @toubatbrian. Right now it is in experimentation stage. The automation will start porting this PR into agents-js automatically.

Notes about this port:

• agents-js does not yet have the AgentConfigUpdate chat item ported (there are existing TODO(parity) markers in agents/src/llm/chat_context.ts and agents/src/voice/agent_activity.ts). To make this PR meaningful in JS, the port will also build the prerequisite: adding the AgentConfigUpdate chat item, wiring it into ChatItem, recording the initial config and updates, and emitting it in the OTLP chat-item proto.
• The JS PR will tag @toubatbrian and @livekit/agent-devs for review.

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