Add realtime silence tool (#18635)

## 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.

codex-rs/tui/src/chatwidget/realtime.rs

@@ -338,6 +338,7 @@ impl ChatWidget {
             RealtimeEvent::ConversationItemAdded(_item) => {}
             RealtimeEvent::ConversationItemDone { .. } => {}
             RealtimeEvent::HandoffRequested(_) => {}
+            RealtimeEvent::NoopRequested(_) => {}
             RealtimeEvent::Error(message) => {
                 self.fail_realtime_conversation(format!("Realtime voice error: {message}"));
             }