Migrate model preset (#7542)

- Introduce `openai_models` in `/core`
- Move `PRESETS` under it
- Move `ModelPreset`, `ModelUpgrade`, `ReasoningEffortPreset`,
`ReasoningEffortPreset`, and `ReasoningEffortPreset` to `protocol`
- Introduce `Op::ListModels` and `EventMsg::AvailableModels`

Next steps:
- migrate `app-server` and `tui` to use the introduced Operation

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

@@ -34,6 +34,7 @@ mod grep_files;
 mod items;
 mod json_result;
 mod list_dir;
+mod list_models;
 mod live_cli;
 mod model_overrides;
 mod model_tools;