fix(core) switching model appends model instructions (#10651)

## Summary
When switching models, we should append the instructions of the new
model to the conversation as a developer message.

## Test
- [x] Adds a unit test

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

@@ -84,6 +84,7 @@ mod live_cli;
 mod live_reload;
 mod model_info_overrides;
 mod model_overrides;
+mod model_switching;
 mod model_tools;
 mod models_cache_ttl;
 mod models_etag_responses;