Consolidate model instructions in ModelMessages (#36787)

## What changed

- Remove `ModelInfo.base_instructions` as an in-memory instruction source and use `model_messages.instructions_template` consistently for bundled, remote, fallback, and overridden model metadata.
- Preserve compatibility by promoting legacy `base_instructions` values when reading model responses and caches, and by including rendered legacy instructions when serializing `ModelsResponse` for older clients.
- Treat templates without instruction variables as literal text and retain the other model-message fields when applying instruction overrides.

This completes the consolidation proposed in https://github.com/openai/codex/pull/31302.

## Testing

- Cover legacy response and cache migration, canonical-template precedence, fallback instructions, personality rendering, overrides, and model switching.

GitOrigin-RevId: 089d986ca5e30da67db2c77a1b6a046d2cff52dc

codex-rs/core/src/session/mod.rs

@@ -632,7 +632,7 @@ impl Session {
         // Resolve base instructions for the session. Priority order:
         // 1. config.base_instructions override
         // 2. conversation history => session_meta.base_instructions
-        // 3. base_instructions for current model
+        // 3. rendered instructions_template for current model
         let model_info = models_manager
             .get_model_info(model.as_str(), &config.to_models_manager_config())
             .await;

codex-rs/core/src/session/tests.rs

@@ -1465,20 +1465,21 @@ async fn get_base_instructions_no_user_content() {
 
     for test_case in test_cases {
         let model_info = model_info_for_slug(test_case.slug, &config);
+        let model_instructions = model_info.get_model_instructions(config.personality);
         if test_case.expects_apply_patch_description {
             assert_eq!(
-                model_info.base_instructions.as_str(),
+                model_instructions.as_str(),
                 prompt_with_apply_patch_instructions
             );
         }
 
         {
             let mut state = session.state.lock().await;
-            state.session_configuration.base_instructions = model_info.base_instructions.clone();
+            state.session_configuration.base_instructions = model_instructions.clone();
         }
 
         let base_instructions = session.get_base_instructions().await;
-        assert_eq!(base_instructions.text, model_info.base_instructions);
+        assert_eq!(base_instructions.text, model_instructions);
     }
 }