Use model catalog approval messages (#31312)

## Why

Approval guidance is currently assembled entirely by the client. Model
Messages V2 needs model catalogs to provide model-specific `on_request`
guidance for both user-reviewed and auto-reviewed approval flows while
retaining the existing generated prompt as a compatibility fallback.

## What changed

- add nullable `on_request` and `on_request_auto_review` catalog
messages
- select the message matching the active approvals reviewer for
`on_request` policies
- replace the complete legacy approval section when the selected catalog
value exists, including support for an empty string that suppresses the
section
- retain legacy rendering when the object or selected key is absent, and
for non-`on_request` policies
- preserve approval messages when base-instruction or personality
overrides clear instruction templates
- refresh permissions instructions when the active model changes
- pass catalog messages through initial and incremental permissions
construction

## Relationship to reviewer persistence

PR #31309 independently persists the approvals reviewer in turn context
and refreshes permissions when that reviewer changes. This PR is based
directly on `main` and does not duplicate that rollout migration; once
both land, reviewer switches will also select and append the new catalog
variant.

## Testing

- `just test -p codex-protocol`
- `just test -p codex-prompts`
- `just test -p codex-models-manager`
- `just test -p codex-core permissions_messages`

codex-rs/models-manager/src/model_info.rs

@@ -54,14 +54,24 @@ pub fn with_config_overrides(mut model: ModelInfo, config: &ModelsManagerConfig)
 
     if let Some(base_instructions) = &config.base_instructions {
         model.base_instructions = base_instructions.clone();
-        model.model_messages = None;
+        clear_instruction_messages(&mut model);
     } else if !config.personality_enabled {
-        model.model_messages = None;
+        clear_instruction_messages(&mut model);
     }
 
     model
 }
 
+fn clear_instruction_messages(model: &mut ModelInfo) {
+    if let Some(model_messages) = &mut model.model_messages {
+        model_messages.instructions_template = None;
+        model_messages.instructions_variables = None;
+        if model_messages.approvals.is_none() {
+            model.model_messages = None;
+        }
+    }
+}
+
 /// Build a minimal fallback model descriptor for missing/unknown slugs.
 pub fn model_info_from_slug(slug: &str) -> ModelInfo {
     warn!("Unknown model {slug} is used. This will use fallback model metadata.");
@@ -119,6 +129,7 @@ fn local_personality_messages_for_slug(slug: &str) -> Option<ModelMessages> {
                 personality_friendly: Some(LOCAL_FRIENDLY_TEMPLATE.to_string()),
                 personality_pragmatic: Some(LOCAL_PRAGMATIC_TEMPLATE.to_string()),
             }),
+            approvals: None,
         }),
         _ => None,
     }