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/protocol/src/protocol.rs

@@ -329,6 +329,12 @@ pub struct RealtimeHandoffRequested {
     pub active_transcript: Vec<RealtimeTranscriptEntry>,
 }
 
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
+pub struct RealtimeNoopRequested {
+    pub call_id: String,
+    pub item_id: String,
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
 pub struct RealtimeInputAudioSpeechStarted {
     pub item_id: Option<String>,
@@ -369,6 +375,7 @@ pub enum RealtimeEvent {
         item_id: String,
     },
     HandoffRequested(RealtimeHandoffRequested),
+    NoopRequested(RealtimeNoopRequested),
     Error(String),
 }