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-ui-admin/app/src/views/ai/plugin/AddPluginModal.vue

@@ -145,12 +145,10 @@ function save() {
           })
           .then(async (res) => {
             if (res.errorCode === 0) {
-              const pluginId =
-                res.data?.id ||
-                res.data ||
-                plainEntity.id ||
-                entity.value.id ||
-                '';
+              const pluginId = res.data?.id || plainEntity.id || entity.value.id;
+              if (!pluginId) {
+                throw new Error('插件保存成功，但未返回插件ID');
+              }
               await syncPluginCategories(pluginId, categoryIds);
               dialogVisible.value = false;
               ElMessage.success($t('message.saveOkMessage'));