✅ test(files): 完善MinioService测试用例

- 修复objectExists测试中错误消息重复设置的问题
- 添加Client模拟实现以确保测试隔离性

src/server/modules/files/__tests__/minio.service.test.ts

@@ -28,6 +28,7 @@ describe('MinioService', () => {
   beforeEach(() => {
     mockClient = new Client({} as any);
     (Client as any).mockClear();
+    (Client as any).mockImplementation(() => mockClient);
 
     minioService = new MinioService();
   });
@@ -381,7 +382,6 @@ describe('MinioService', () => {
 
     it('should return false when object not found', async () => {
       const error = new Error('Object not found');
-      error.message = 'The specified key does not exist';
       vi.mocked(mockClient.statObject).mockRejectedValue(error);
 
       const result = await minioService.objectExists('test-bucket', 'nonexistent.txt');