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/core/src/models_manager/model_info.rs

@@ -69,6 +69,7 @@ pub(crate) fn model_info_from_slug(slug: &str) -> ModelInfo {
         visibility: ModelVisibility::None,
         supported_in_api: true,
         priority: 99,
+        availability_nux: None,
         upgrade: None,
         base_instructions: BASE_INSTRUCTIONS.to_string(),
         model_messages: local_personality_messages_for_slug(slug),