Add model availability NUX metadata (#12972)

- replace show_nux with structured availability_nux model metadata
- expose availability NUX data through the app-server model API
- update shared fixtures and tests for the new field

codex-rs/tui/src/chatwidget/tests.rs

@@ -6077,6 +6077,7 @@ async fn model_picker_hides_show_in_picker_false_models_from_cache() {
         is_default: false,
         upgrade: None,
         show_in_picker,
+        availability_nux: None,
         supported_in_api: true,
         input_modalities: default_input_modalities(),
     };
@@ -6345,6 +6346,7 @@ async fn single_reasoning_option_skips_selection() {
         is_default: false,
         upgrade: None,
         show_in_picker: true,
+        availability_nux: None,
         supported_in_api: true,
         input_modalities: default_input_modalities(),
     };