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/src/rollout/policy.rs

@@ -51,6 +51,7 @@ pub(crate) fn should_persist_event_msg(ev: &EventMsg) -> bool {
         | EventMsg::Warning(_)
         | EventMsg::TaskStarted(_)
         | EventMsg::TaskComplete(_)
+        | EventMsg::ListModelsResponse(_)
         | EventMsg::AgentMessageDelta(_)
         | EventMsg::AgentReasoningDelta(_)
         | EventMsg::AgentReasoningRawContentDelta(_)