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/codex-api/tests/models_integration.rs

@@ -77,6 +77,7 @@ async fn models_client_hits_models_endpoint() {
             priority: 1,
             upgrade: None,
             base_instructions: "base instructions".to_string(),
+            model_instructions_template: None,
             supports_reasoning_summaries: false,
             support_verbosity: false,
             default_verbosity: None,