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/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,