Summary

Adds a second realtime v2 function tool, remain_silent, so the realtime model has an explicit non-speaking action when the collaboration mode or latest context says it should not answer aloud. This is stacked on #18597.

Design

• Advertise remain_silent alongside background_agent in realtime v2 conversational sessions.
• Parse remain_silent function calls into a typed RealtimeEvent::NoopRequested event.
• Have core answer that function call with an empty function_call_output and deliberately avoid response.create, so no follow-up realtime response is requested.
• Keep the event hidden from app-server/TUI surfaces; it is operational plumbing, not user-visible conversation content.