Queue Realtime V2 response.create while active (#17306)

Builds on #17264.

- queues Realtime V2 `response.create` while an active response is open,
then flushes it after `response.done` or `response.cancelled`
- requests `response.create` after background agent final output and
steering acknowledgements
- adds app-server integration coverage for all `response.create` paths

Validation:
- `just fmt`
- `cargo check -p codex-app-server --tests`
- `git diff --check`
- CI green

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/codex-api/src/endpoint/realtime_websocket/methods.rs

@@ -419,7 +419,9 @@ impl RealtimeWebsocketEvents {
             }
             RealtimeEvent::SessionUpdated { .. }
             | RealtimeEvent::AudioOut(_)
+            | RealtimeEvent::ResponseCreated(_)
             | RealtimeEvent::ResponseCancelled(_)
+            | RealtimeEvent::ResponseDone(_)
             | RealtimeEvent::ConversationItemAdded(_)
             | RealtimeEvent::ConversationItemDone { .. }
             | RealtimeEvent::Error(_) => {}
@@ -724,6 +726,8 @@ mod tests {
     use codex_protocol::protocol::RealtimeHandoffRequested;
     use codex_protocol::protocol::RealtimeInputAudioSpeechStarted;
     use codex_protocol::protocol::RealtimeResponseCancelled;
+    use codex_protocol::protocol::RealtimeResponseCreated;
+    use codex_protocol::protocol::RealtimeResponseDone;
     use codex_protocol::protocol::RealtimeVoice;
     use http::HeaderValue;
     use pretty_assertions::assert_eq;
@@ -999,7 +1003,9 @@ mod tests {
 
         assert_eq!(
             parse_realtime_event(payload.as_str(), RealtimeEventParser::RealtimeV2),
-            None
+            Some(RealtimeEvent::ResponseDone(RealtimeResponseDone {
+                response_id: None
+            }))
         );
     }
 
@@ -1013,10 +1019,9 @@ mod tests {
 
         assert_eq!(
             parse_realtime_event(payload.as_str(), RealtimeEventParser::RealtimeV2),
-            Some(RealtimeEvent::ConversationItemAdded(json!({
-                "type": "response.created",
-                "response": {"id": "resp_created_1"}
-            })))
+            Some(RealtimeEvent::ResponseCreated(RealtimeResponseCreated {
+                response_id: Some("resp_created_1".to_string())
+            }))
         );
     }