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/models-manager/src/model_info.rs

@@ -50,22 +50,48 @@ pub fn with_config_overrides(mut model: ModelInfo, config: &ModelsManagerConfig)
     }
 
     if let Some(base_instructions) = &config.base_instructions {
-        model.base_instructions = base_instructions.clone();
-        clear_instruction_messages(&mut model);
+        let model_messages = model.model_messages.get_or_insert(ModelMessages {
+            instructions_template: None,
+            instructions_variables: None,
+            approvals: None,
+            collaboration_modes: None,
+            auto_review: None,
+            permissions: None,
+            token_budget: None,
+        });
+        model_messages.instructions_template = Some(base_instructions.clone());
+        model_messages.instructions_variables = None;
     } else {
-        if config.personality_enabled && config.personality == Some(Personality::None) {
-            model.base_instructions = strip_personality_section(model.base_instructions);
-            if let Some(instructions_template) = model
+        if config.personality_enabled
+            && config.personality == Some(Personality::None)
+            && let Some(instructions_template) = model
                 .model_messages
                 .as_mut()
                 .and_then(|messages| messages.instructions_template.as_mut())
-            {
-                *instructions_template =
-                    strip_personality_section(std::mem::take(instructions_template));
-            }
+        {
+            *instructions_template =
+                strip_personality_section(std::mem::take(instructions_template));
         }
-        if !config.personality_enabled {
-            clear_instruction_messages(&mut model);
+        let uses_local_personality_template = model.used_fallback_model_metadata
+            && matches!(
+                model.slug.as_str(),
+                "gpt-5.2-codex" | "exp-codex-personality"
+            );
+        if !config.personality_enabled
+            && let Some(model_messages) = model.model_messages.as_mut()
+        {
+            if uses_local_personality_template {
+                model_messages.instructions_template = Some(BASE_INSTRUCTIONS.to_string());
+            } else {
+                let personality_default = model_messages
+                    .get_personality_message(/*personality*/ None)
+                    .unwrap_or_default();
+                if let Some(instructions_template) = model_messages.instructions_template.as_mut() {
+                    *instructions_template = instructions_template
+                        .replace(PERSONALITY_PLACEHOLDER, &personality_default);
+                }
+            }
+            model_messages.instructions_variables = None;
         }
     }
 
@@ -108,21 +134,6 @@ fn is_h1_heading(line: &str) -> bool {
     rest.is_empty() || rest.starts_with(' ') || rest.starts_with('\t')
 }
 
-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_messages.collaboration_modes.is_none()
-            && model_messages.auto_review.is_none()
-            && model_messages.permissions.is_none()
-            && model_messages.token_budget.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.");
@@ -141,8 +152,7 @@ pub fn model_info_from_slug(slug: &str) -> ModelInfo {
         default_service_tier: None,
         availability_nux: None,
         upgrade: None,
-        base_instructions: BASE_INSTRUCTIONS.to_string(),
-        model_messages: local_personality_messages_for_slug(slug),
+        model_messages: Some(local_model_messages_for_slug(slug)),
         include_skills_usage_instructions: false,
         supports_reasoning_summary_parameter: true,
         default_reasoning_summary: ReasoningSummary::Auto,
@@ -169,9 +179,9 @@ pub fn model_info_from_slug(slug: &str) -> ModelInfo {
     }
 }
 
-fn local_personality_messages_for_slug(slug: &str) -> Option<ModelMessages> {
+fn local_model_messages_for_slug(slug: &str) -> ModelMessages {
     match slug {
-        "gpt-5.2-codex" | "exp-codex-personality" => Some(ModelMessages {
+        "gpt-5.2-codex" | "exp-codex-personality" => ModelMessages {
             instructions_template: Some(format!(
                 "{DEFAULT_PERSONALITY_HEADER}\n\n{PERSONALITY_PLACEHOLDER}\n\n{BASE_INSTRUCTIONS}"
             )),
@@ -185,8 +195,16 @@ fn local_personality_messages_for_slug(slug: &str) -> Option<ModelMessages> {
             auto_review: None,
             permissions: None,
             token_budget: None,
-        }),
-        _ => None,
+        },
+        _ => ModelMessages {
+            instructions_template: Some(BASE_INSTRUCTIONS.to_string()),
+            instructions_variables: None,
+            approvals: None,
+            collaboration_modes: None,
+            auto_review: None,
+            permissions: None,
+            token_budget: None,
+        },
     }
 }