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/app-server/tests/common/models_cache.rs

@@ -27,6 +27,7 @@ fn preset_to_info(preset: &ModelPreset, priority: i32) -> ModelInfo {
         priority,
         upgrade: preset.upgrade.as_ref().map(|u| u.into()),
         base_instructions: "base instructions".to_string(),
+        model_instructions_template: None,
         supports_reasoning_summaries: false,
         support_verbosity: false,
         default_verbosity: None,