Support model catalog templates for Guardian policy prompts (#33177)

## What changed

- Add an optional `policy_template` field to auto-review model messages.
- Build Guardian instructions from the catalog template when present, while
  preserving the existing precedence for managed and catalog policy content.
- Fall back independently to the bundled template and bundled policy, and
  preserve explicit empty catalog values.

## Testing

- Cover missing and empty template deserialization, policy precedence, bundled
  fallbacks, and Guardian prewarm instructions.

GitOrigin-RevId: 93116dadbc9bf40527882844a428caed4159fff6

codex-rs/protocol/src/openai_models.rs

@@ -518,6 +518,7 @@ pub struct ApprovalMessages {
 #[derive(Debug, Serialize, Deserialize, Clone, PartialEq, Eq, TS, JsonSchema)]
 pub struct AutoReviewMessages {
     pub policy: Option<String>,
+    pub policy_template: Option<String>,
 }
 
 #[derive(Debug, Serialize, Deserialize, Clone, PartialEq, Eq, TS, JsonSchema)]
@@ -777,6 +778,46 @@ mod tests {
         );
     }
 
+    #[test]
+    fn auto_review_messages_preserve_missing_and_empty_template_values() {
+        let missing_template: ModelMessages = from_str(
+            r#"{
+                "instructions_template": null,
+                "instructions_variables": null,
+                "auto_review": {
+                    "policy": "policy"
+                }
+            }"#,
+        )
+        .expect("auto-review messages should deserialize without a policy template");
+        let empty_template: ModelMessages = from_str(
+            r#"{
+                "instructions_template": null,
+                "instructions_variables": null,
+                "auto_review": {
+                    "policy": "policy",
+                    "policy_template": ""
+                }
+            }"#,
+        )
+        .expect("auto-review messages should deserialize with an empty policy template");
+
+        assert_eq!(
+            missing_template.auto_review,
+            Some(AutoReviewMessages {
+                policy: Some("policy".to_string()),
+                policy_template: None,
+            })
+        );
+        assert_eq!(
+            empty_template.auto_review,
+            Some(AutoReviewMessages {
+                policy: Some("policy".to_string()),
+                policy_template: Some(String::new()),
+            })
+        );
+    }
+
     #[test]
     fn permission_messages_preserve_missing_and_empty_values() {
         let messages: ModelMessages = from_str(