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/protocol/src/protocol.rs

@@ -313,6 +313,16 @@ pub struct RealtimeResponseCancelled {
     pub response_id: Option<String>,
 }
 
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
+pub struct RealtimeResponseCreated {
+    pub response_id: Option<String>,
+}
+
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
+pub struct RealtimeResponseDone {
+    pub response_id: Option<String>,
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
 pub enum RealtimeEvent {
     SessionUpdated {
@@ -323,7 +333,9 @@ pub enum RealtimeEvent {
     InputTranscriptDelta(RealtimeTranscriptDelta),
     OutputTranscriptDelta(RealtimeTranscriptDelta),
     AudioOut(RealtimeAudioFrame),
+    ResponseCreated(RealtimeResponseCreated),
     ResponseCancelled(RealtimeResponseCancelled),
+    ResponseDone(RealtimeResponseDone),
     ConversationItemAdded(Value),
     ConversationItemDone {
         item_id: String,