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/protocol/src/lib.rs

@@ -8,6 +8,7 @@ pub mod items;
 pub mod message_history;
 pub mod models;
 pub mod num_format;
+pub mod openai_models;
 pub mod parse_command;
 pub mod plan_tool;
 pub mod protocol;