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/exec/src/event_processor_with_human_output.rs

@@ -583,6 +583,7 @@ impl EventProcessor for EventProcessorWithHumanOutput {
             | EventMsg::ReasoningContentDelta(_)
             | EventMsg::ReasoningRawContentDelta(_)
             | EventMsg::UndoCompleted(_)
+            | EventMsg::ListModelsResponse(_)
             | EventMsg::UndoStarted(_) => {}
         }
         CodexStatus::Running