[app-server] model/list API (#5382)

Adds a `model/list` paginated API that returns the list of models
supported by Codex.

codex-rs/app-server/src/models.rs

@@ -0,0 +1,38 @@
+use codex_app_server_protocol::Model;
+use codex_app_server_protocol::ReasoningEffortOption;
+use codex_common::model_presets::ModelPreset;
+use codex_common::model_presets::ReasoningEffortPreset;
+use codex_common::model_presets::builtin_model_presets;
+
+pub fn supported_models() -> Vec<Model> {
+    builtin_model_presets(None)
+        .into_iter()
+        .map(model_from_preset)
+        .collect()
+}
+
+fn model_from_preset(preset: ModelPreset) -> Model {
+    Model {
+        id: preset.id.to_string(),
+        model: preset.model.to_string(),
+        display_name: preset.display_name.to_string(),
+        description: preset.description.to_string(),
+        supported_reasoning_efforts: reasoning_efforts_from_preset(
+            preset.supported_reasoning_efforts,
+        ),
+        default_reasoning_effort: preset.default_reasoning_effort,
+        is_default: preset.is_default,
+    }
+}
+
+fn reasoning_efforts_from_preset(
+    efforts: &'static [ReasoningEffortPreset],
+) -> Vec<ReasoningEffortOption> {
+    efforts
+        .iter()
+        .map(|preset| ReasoningEffortOption {
+            reasoning_effort: preset.effort,
+            description: preset.description.to_string(),
+        })
+        .collect()
+}