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/app-server/src/bespoke_event_handling.rs

@@ -445,6 +445,7 @@ pub(crate) async fn apply_bespoke_event_handling(
                             )
                             .await;
                     }
+                    RealtimeEvent::ResponseCreated(_) => {}
                     RealtimeEvent::ResponseCancelled(event) => {
                         let notification = ThreadRealtimeItemAddedNotification {
                             thread_id: conversation_id.to_string(),
@@ -459,6 +460,7 @@ pub(crate) async fn apply_bespoke_event_handling(
                             ))
                             .await;
                     }
+                    RealtimeEvent::ResponseDone(_) => {}
                     RealtimeEvent::ConversationItemAdded(item) => {
                         let notification = ThreadRealtimeItemAddedNotification {
                             thread_id: conversation_id.to_string(),