Propagate safety buffering events to app-server clients (#29371)

Responses API safety buffering metadata currently stops at the transport
boundary, so app-server clients cannot render the in-progress safety
review state.

This change:
- decodes and deduplicates `safety_buffering` metadata from Responses
API SSE and WebSocket events without suppressing the original response
event
- emits a typed core event containing the requested model plus backend
use cases and reasons
- forwards that event as `turn/safetyBuffering/updated` through
app-server v2 and updates generated protocol schemas
- keeps the side-channel event out of persisted rollouts and turn timing

This supports the Codex Apps buffering UX and depends on the Responses
API backend work in https://github.com/openai/openai/pull/1044569 and
https://github.com/openai/openai/pull/1044571.

Validation:
- focused `codex-core` safety-buffering integration test passes
- `cargo check -p codex-core -p codex-app-server -p
codex-app-server-protocol`
- `just fix -p codex-api -p codex-protocol -p codex-core -p
codex-app-server-protocol -p codex-app-server -p codex-rollout -p
codex-rollout-trace -p codex-otel`
- `just fmt`
- broad package test run: 4,430/4,492 passed; 62 unrelated
local-environment/concurrency failures involved unavailable test
binaries, MCP subprocess setup, and app-server timeouts

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -1655,6 +1655,7 @@ server_notification_definitions! {
     ModelVerification => "model/verification" (v2::ModelVerificationNotification),
     #[experimental("turn/moderationMetadata")]
     TurnModerationMetadata => "turn/moderationMetadata" (v2::TurnModerationMetadataNotification),
+    ModelSafetyBufferingUpdated => "model/safetyBuffering/updated" (v2::ModelSafetyBufferingUpdatedNotification),
     Warning => "warning" (v2::WarningNotification),
     GuardianWarning => "guardianWarning" (v2::GuardianWarningNotification),
     DeprecationNotice => "deprecationNotice" (v2::DeprecationNoticeNotification),
@@ -3347,6 +3348,33 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn serialize_model_safety_buffering_updated_notification() -> Result<()> {
+        let notification = ServerNotification::ModelSafetyBufferingUpdated(
+            v2::ModelSafetyBufferingUpdatedNotification {
+                thread_id: "thr_123".to_string(),
+                turn_id: "turn_123".to_string(),
+                model: "gpt-5.4".to_string(),
+                use_cases: vec!["cyber".to_string()],
+                reasons: vec!["user_risk".to_string()],
+            },
+        );
+        assert_eq!(
+            json!({
+                "method": "model/safetyBuffering/updated",
+                "params": {
+                    "threadId": "thr_123",
+                    "turnId": "turn_123",
+                    "model": "gpt-5.4",
+                    "useCases": ["cyber"],
+                    "reasons": ["user_risk"]
+                }
+            }),
+            serde_json::to_value(&notification)?,
+        );
+        Ok(())
+    }
+
     #[test]
     fn serialize_thread_realtime_output_audio_delta_notification() -> Result<()> {
         let notification = ServerNotification::ThreadRealtimeOutputAudioDelta(

codex-rs/app-server-protocol/src/protocol/v2/model.rs

@@ -162,3 +162,14 @@ pub struct TurnModerationMetadataNotification {
     pub turn_id: String,
     pub metadata: JsonValue,
 }
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ModelSafetyBufferingUpdatedNotification {
+    pub thread_id: String,
+    pub turn_id: String,
+    pub model: String,
+    pub use_cases: Vec<String>,
+    pub reasons: Vec<String>,
+}