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/openai_models.rs

@@ -0,0 +1,80 @@
+use std::collections::HashMap;
+
+use schemars::JsonSchema;
+use serde::Deserialize;
+use serde::Serialize;
+use strum_macros::Display;
+use strum_macros::EnumIter;
+use ts_rs::TS;
+
+/// See https://platform.openai.com/docs/guides/reasoning?api-mode=responses#get-started-with-reasoning
+#[derive(
+    Debug,
+    Serialize,
+    Deserialize,
+    Default,
+    Clone,
+    Copy,
+    PartialEq,
+    Eq,
+    Display,
+    JsonSchema,
+    TS,
+    EnumIter,
+    Hash,
+)]
+#[serde(rename_all = "lowercase")]
+#[strum(serialize_all = "lowercase")]
+pub enum ReasoningEffort {
+    None,
+    Minimal,
+    Low,
+    #[default]
+    Medium,
+    High,
+    XHigh,
+}
+
+#[derive(Debug, Clone, Deserialize, Serialize, TS, JsonSchema)]
+pub struct AvailableModelsEvent {
+    pub models: Vec<ModelPreset>,
+}
+
+/// A reasoning effort option that can be surfaced for a model.
+#[derive(Debug, Clone, Deserialize, Serialize, TS, JsonSchema, PartialEq)]
+pub struct ReasoningEffortPreset {
+    /// Effort level that the model supports.
+    pub effort: ReasoningEffort,
+    /// Short human description shown next to the effort in UIs.
+    pub description: String,
+}
+
+#[derive(Debug, Clone, Deserialize, Serialize, TS, JsonSchema, PartialEq)]
+pub struct ModelUpgrade {
+    pub id: String,
+    pub reasoning_effort_mapping: Option<HashMap<ReasoningEffort, ReasoningEffort>>,
+    pub migration_config_key: String,
+}
+
+/// Metadata describing a Codex-supported model.
+#[derive(Debug, Clone, Deserialize, Serialize, TS, JsonSchema, PartialEq)]
+pub struct ModelPreset {
+    /// Stable identifier for the preset.
+    pub id: String,
+    /// Model slug (e.g., "gpt-5").
+    pub model: String,
+    /// Display name shown in UIs.
+    pub display_name: String,
+    /// Short human description shown in UIs.
+    pub description: String,
+    /// Reasoning effort applied when none is explicitly chosen.
+    pub default_reasoning_effort: ReasoningEffort,
+    /// Supported reasoning effort options.
+    pub supported_reasoning_efforts: Vec<ReasoningEffortPreset>,
+    /// Whether this is the default model for new users.
+    pub is_default: bool,
+    /// recommended upgrade model
+    pub upgrade: Option<ModelUpgrade>,
+    /// Whether this preset should appear in the picker UI.
+    pub show_in_picker: bool,
+}