feat: preserve user-selected catalog model in Codex Chat requests

When Codex client sends a model from the catalog (e.g., user selected via /model),
preserve that choice instead of always replacing with config.toml's default model.

- Check if request model exists in modelCatalog before falling back to config model
- Remove CODEX_CHAT_CLIENT_MODEL constant (no longer needed)
- Add test coverage for catalog model preservation