fix(frontend): guard response.error.includes against non-string error values

If the API returns a non-string error (e.g. numeric HTTP status code),
calling .includes() directly on it throws "N.includes is not a function".
Convert to string via JSON.stringify first.

Fixes save button crash on rules/new page.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/routes/rules.new.tsx

@@ -855,10 +855,11 @@ export default function RuleNew() {
         }
 
         if (response.error) {
-          if (response.error.includes('evaluation_points_code_key')) {
+          const errorStr = typeof response.error === 'string' ? response.error : JSON.stringify(response.error);
+          if (errorStr.includes('evaluation_points_code_key')) {
             toastService.error('在基本信息中：评查点编码已存在，请修改后保存。');
           } else {
-            toastService.error(`系统繁忙: ${response.error}`);
+            toastService.error(`系统繁忙: ${errorStr}`);
           }
           setIsLoading(false);
         } else if (response.data) {