[codex] Read retry model from buffering events (#31262)

## Summary

- deserialize `retry_model` from streamed `safety_buffering` payloads
- preserve the existing downstream faster-model API and legacy header
fallback
- update SSE, WebSocket, and end-to-end safety-buffering coverage

## Root cause

Follow-up to #31064. The Responses API emits the retry target as
`retry_model`, but the client was looking for `faster_model`, so the
payload value was ignored in favor of the compatibility fallback.

## Behavior

A non-null `retry_model` from the buffering payload takes precedence. An
explicit null leaves the retry target unset, while an omitted field
continues to fall back to the existing response header.

## Validation

- `just test -p codex-api` (135 tests)
- `just test -p codex-core safety_buffering` (2 tests)
- `just fix -p codex-api`
- `just fmt`
- `git diff --check`

codex-rs/codex-api/src/common.rs

@@ -123,6 +123,7 @@ pub struct SafetyBuffering {
     pub reasons: Vec<String>,
     #[serde(skip)]
     pub show_buffering_ui: bool,
+    #[serde(rename = "retry_model")]
     pub faster_model: Option<String>,
 }
 

codex-rs/codex-api/src/endpoint/responses_websocket.rs

@@ -1061,7 +1061,7 @@ mod tests {
             "safety_buffering": {
                 "use_cases": ["cyber"],
                 "reasons": ["user_risk"],
-                "faster_model": "gpt-fast-wire"
+                "retry_model": "gpt-fast-wire"
             }
         }))
         .expect("deserialize safety buffering event");

codex-rs/codex-api/src/sse/responses.rs

@@ -236,10 +236,10 @@ impl ResponsesStreamEvent {
         treatment: &SafetyBufferingTreatment,
     ) -> Option<SafetyBuffering> {
         let value = self.safety_buffering.as_ref()?;
-        let faster_model_present = value.as_object()?.contains_key("faster_model");
+        let retry_model_present = value.as_object()?.contains_key("retry_model");
         let mut buffering: SafetyBuffering = serde_json::from_value(value.clone()).ok()?;
         buffering.show_buffering_ui = true;
-        if !faster_model_present {
+        if !retry_model_present {
             buffering.faster_model.clone_from(&treatment.faster_model);
         }
         Some(buffering)
@@ -1405,7 +1405,7 @@ mod tests {
                 "safety_buffering": {
                     "use_cases": ["cyber"],
                     "reasons": ["user_risk"],
-                    "faster_model": "gpt-fast-wire"
+                    "retry_model": "gpt-fast-wire"
                 }
             }),
             json!({
@@ -1453,12 +1453,12 @@ mod tests {
     }
 
     #[test]
-    fn safety_buffering_prefers_wire_faster_model_and_only_falls_back_when_omitted() {
+    fn safety_buffering_prefers_wire_retry_model_and_only_falls_back_when_omitted() {
         let treatment = SafetyBufferingTreatment {
             faster_model: Some("gpt-fast-header".to_string()),
         };
 
-        for (faster_model, expected_faster_model) in [
+        for (retry_model, expected_faster_model) in [
             (None, Some("gpt-fast-header")),
             (Some(Value::Null), None),
             (Some(json!("gpt-fast-wire")), Some("gpt-fast-wire")),
@@ -1470,8 +1470,8 @@ mod tests {
                     "reasons": ["user_risk"]
                 }
             });
-            if let Some(faster_model) = faster_model {
+            if let Some(retry_model) = retry_model {
-                event["safety_buffering"]["faster_model"] = faster_model;
+                event["safety_buffering"]["retry_model"] = retry_model;
             }
             let event: ResponsesStreamEvent =
                 serde_json::from_value(event).expect("deserialize safety buffering event");

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

@@ -83,7 +83,7 @@ async fn emits_safety_buffering_with_the_responses_api_model_without_header_gati
     created["safety_buffering"] = json!({
         "use_cases": ["cyber"],
         "reasons": ["policy-check"],
-        "faster_model": FASTER_MODEL,
+        "retry_model": FASTER_MODEL,
     });
     mount_response_once(
         &server,