fix(admin): 修复 Key 自动获取模型时 allowed_models 同步逻辑

- 关闭自动获取上游模型时清空 allowed_models
- 开启自动获取上游模型时立即拉取并覆盖 allowed_models
- 增加模型覆盖提示并补充相关回归测试

apps/aether-gateway/src/model_fetch/mod.rs

@@ -4,4 +4,6 @@ mod tests;
 
 pub(crate) use aether_model_fetch::ModelFetchRunSummary;
 pub(crate) use runtime::state::ModelFetchRuntimeState;
-pub(crate) use runtime::{perform_model_fetch_once, spawn_model_fetch_worker};
+pub(crate) use runtime::{
+    perform_model_fetch_for_key, perform_model_fetch_once, spawn_model_fetch_worker,
+};