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/tui/src/chatwidget/realtime.rs

@@ -340,7 +340,9 @@ impl ChatWidget {
                 ev.payload,
                 RealtimeEvent::AudioOut(_)
                     | RealtimeEvent::InputAudioSpeechStarted(_)
+                    | RealtimeEvent::ResponseCreated(_)
                     | RealtimeEvent::ResponseCancelled(_)
+                    | RealtimeEvent::ResponseDone(_)
             )
         {
             return;
@@ -353,7 +355,9 @@ impl ChatWidget {
             RealtimeEvent::InputTranscriptDelta(_) => {}
             RealtimeEvent::OutputTranscriptDelta(_) => {}
             RealtimeEvent::AudioOut(frame) => self.enqueue_realtime_audio_out(&frame),
+            RealtimeEvent::ResponseCreated(_) => {}
             RealtimeEvent::ResponseCancelled(_) => self.interrupt_realtime_audio_playback(),
+            RealtimeEvent::ResponseDone(_) => {}
             RealtimeEvent::ConversationItemAdded(_item) => {}
             RealtimeEvent::ConversationItemDone { .. } => {}
             RealtimeEvent::HandoffRequested(_) => {}