fix: 修复插件保存后的分类关联

- 插件保存接口返回实体以便前端拿到真实插件 ID

- 分类关联更新改为按差异增删并补充事务保护

- 新建插件后缺失 ID 时明确抛出错误

easyflow-api/easyflow-api-admin/src/main/java/tech/easyflow/admin/controller/ai/PluginController.java

@@ -40,7 +40,7 @@ public class PluginController extends BaseCurdController<PluginService, Plugin>
 
     @PostMapping("/plugin/save")
     @SaCheckPermission("/api/v1/plugin/save")
-    public Result<Boolean> savePlugin(@JsonBody Plugin plugin){
+    public Result<Plugin> savePlugin(@JsonBody Plugin plugin){
 
         return Result.ok(pluginService.savePlugin(plugin));
     }