feat(core) ModelInfo.model_instructions_template (#9597)

## Summary
#9555 is the start of a rename, so I'm starting to standardize here.
Sets up `model_instructions` templating with a strongly-typed object for
injecting a personality block into the model instructions.

## Testing
- [x] Added tests
- [x] Ran locally

codex-rs/core/tests/suite/mod.rs

@@ -48,6 +48,7 @@ mod models_etag_responses;
 mod otel;
 mod pending_input;
 mod permissions_messages;
+mod personality;
 mod prompt_caching;
 mod quota_exceeded;
 mod read_file;

codex-rs/core/tests/suite/models_cache_ttl.rs

@@ -174,6 +174,7 @@ fn test_remote_model(slug: &str, priority: i32) -> ModelInfo {
         priority,
         upgrade: None,
         base_instructions: "base instructions".to_string(),
+        model_instructions_template: None,
         supports_reasoning_summaries: false,
         support_verbosity: false,
         default_verbosity: None,

codex-rs/core/tests/suite/personality.rs

@@ -4,20 +4,31 @@ use core_test_support::load_default_config_for_test;
 use pretty_assertions::assert_eq;
 use tempfile::TempDir;
 
-const BASE_INSTRUCTIONS_TEMPLATE: &str = include_str!(
-    "../../templates/model_instructions/gpt-5.2-codex_instructions_template.md"
-);
-const FRIENDLY_PERSONALITY: &str = include_str!("../../templates/personalities/friendly.md");
-
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
-async fn model_personality_updates_base_instructions() {
+async fn model_personality_does_not_mutate_base_instructions_without_template() {
     let codex_home = TempDir::new().expect("create temp dir");
     let mut config = load_default_config_for_test(&codex_home).await;
     config.model_personality = Some(Personality::Friendly);
 
-    let model_info = ModelsManager::construct_model_info_offline("gpt-5.2-codex", &config);
-    let expected =
-        BASE_INSTRUCTIONS_TEMPLATE.replace("{{ personality_message }}", FRIENDLY_PERSONALITY);
-
-    assert_eq!(model_info.base_instructions, expected);
+    let model_info = ModelsManager::construct_model_info_offline("gpt-5.1", &config);
+    assert_eq!(
+        model_info.get_model_instructions(config.model_personality),
+        model_info.base_instructions
+    );
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn base_instructions_override_disables_personality_template() {
+    let codex_home = TempDir::new().expect("create temp dir");
+    let mut config = load_default_config_for_test(&codex_home).await;
+    config.model_personality = Some(Personality::Friendly);
+    config.base_instructions = Some("override instructions".to_string());
+
+    let model_info = ModelsManager::construct_model_info_offline("gpt-5.2-codex", &config);
+
+    assert_eq!(model_info.base_instructions, "override instructions");
+    assert_eq!(
+        model_info.get_model_instructions(config.model_personality),
+        "override instructions"
+    );
 }

codex-rs/core/tests/suite/remote_models.rs

@@ -79,6 +79,7 @@ async fn remote_models_remote_model_uses_unified_exec() -> Result<()> {
         priority: 1,
         upgrade: None,
         base_instructions: "base instructions".to_string(),
+        model_instructions_template: None,
         supports_reasoning_summaries: false,
         support_verbosity: false,
         default_verbosity: None,
@@ -312,6 +313,7 @@ async fn remote_models_apply_remote_base_instructions() -> Result<()> {
         priority: 1,
         upgrade: None,
         base_instructions: remote_base.to_string(),
+        model_instructions_template: None,
         supports_reasoning_summaries: false,
         support_verbosity: false,
         default_verbosity: None,
@@ -782,6 +784,7 @@ fn test_remote_model_with_policy(
         priority,
         upgrade: None,
         base_instructions: "base instructions".to_string(),
+        model_instructions_template: None,
         supports_reasoning_summaries: false,
         support_verbosity: false,
         default_verbosity: None,