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;