[codex] Read buffering metadata from response events (#31064)

## Summary

- read optional faster-model metadata from streamed buffering payloads
- use the buffering payload itself to determine whether buffering UI
should be shown
- retain the existing header value as a compatibility fallback when the
payload omits the field

## Behavior

An object-valued buffering signal now enables the buffering UI. The
response event's faster-model field takes precedence when present, while
omitted fields fall back to existing response metadata. An explicit null
leaves the retry target unset.

## Validation

- `just test -p codex-api`
- `just fix -p codex-api`
- `cargo fmt --all -- --check`
- `git diff --check`

codex-rs/core/tests/suite/safety_buffering.rs

@@ -19,7 +19,7 @@ use serde_json::json;
 const FASTER_MODEL: &str = "faster-model";
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
-async fn emits_safety_buffering_with_the_requested_model() -> anyhow::Result<()> {
+async fn emits_safety_buffering_with_the_header_fallback_model() -> anyhow::Result<()> {
     skip_if_no_network!(Ok(()));
 
     let server = start_mock_server().await;
@@ -72,3 +72,58 @@ async fn emits_safety_buffering_with_the_requested_model() -> anyhow::Result<()>
 
     Ok(())
 }
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn emits_safety_buffering_with_the_responses_api_model_without_header_gating()
+-> anyhow::Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    let mut created = ev_response_created("resp-1");
+    created["safety_buffering"] = json!({
+        "use_cases": ["cyber"],
+        "reasons": ["policy-check"],
+        "faster_model": FASTER_MODEL,
+    });
+    mount_response_once(
+        &server,
+        sse_response(sse(vec![created, ev_completed("resp-1")])),
+    )
+    .await;
+
+    let test = test_codex().build(&server).await?;
+    test.codex
+        .submit(Op::UserInput {
+            items: vec![UserInput::Text {
+                text: "Check this request".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+            responsesapi_client_metadata: None,
+            additional_context: Default::default(),
+            thread_settings: Default::default(),
+        })
+        .await?;
+
+    let event = wait_for_event_match(&test.codex, |event| match event {
+        EventMsg::SafetyBuffering(event) => Some(event.clone()),
+        _ => None,
+    })
+    .await;
+    assert_eq!(
+        event,
+        SafetyBufferingEvent {
+            model: test.session_configured.model.clone(),
+            use_cases: vec!["cyber".to_string()],
+            reasons: vec!["policy-check".to_string()],
+            show_buffering_ui: true,
+            faster_model: Some(FASTER_MODEL.to_string()),
+        }
+    );
+    wait_for_event(&test.codex, |event| {
+        matches!(event, EventMsg::TurnComplete(_))
+    })
+    .await;
+
+    Ok(())
+}