Source Guardian v2 defaults from the model catalog (#38990)

## What changed

- Add optional `guardian_v2` model-message defaults for classifier instructions,
  review thresholds, reasoning effort, transcript selection and limits, and
  action and compaction token limits.
- Apply those defaults when Guardian v2 samples a tool call, while preserving
  explicit `[features.guardianv2]` settings over catalog values.
- Preserve the new catalog configuration when applying model overrides.

## Testing

- Cover model-catalog serialization and model override behavior.
- Verify that sampling uses catalog defaults while retaining local overrides.

GitOrigin-RevId: 4ae9ec235c647acbca05a20b7c4b87a4e1331161

codex-rs/core/src/guardian/review_session.rs

@@ -1836,6 +1836,7 @@ mod tests {
             permissions: None,
             multi_agent: None,
             token_budget: None,
+            guardian_v2: None,
         };
 
         let guardian_config = build_guardian_review_session_config(
@@ -1871,6 +1872,7 @@ mod tests {
             permissions: None,
             multi_agent: None,
             token_budget: None,
+            guardian_v2: None,
         };
 
         let guardian_config = build_guardian_review_session_config(
@@ -1914,6 +1916,7 @@ mod tests {
             permissions: None,
             multi_agent: None,
             token_budget: None,
+            guardian_v2: None,
         };
 
         let guardian_config = build_guardian_review_session_config(