style(auth): 优化auth模块代码规范

范围: src/business/auth/
- register.service.ts: 清理未使用的导入TokenPair，增强userId非空验证
- register.service.spec.ts: 清理未使用的变量apiKeySecurityService

src/business/auth/register.service.spec.ts

@@ -7,12 +7,13 @@
  * - 测试Zulip账号集成
  * 
  * 最近修改：
+ * - 2026-01-15: 代码规范优化 - 清理未使用的变量apiKeySecurityService (修改者: moyin)
  * - 2026-01-12: 代码分离 - 从login.service.spec.ts中分离注册相关测试
  * 
  * @author moyin
- * @version 1.0.0
+ * @version 1.0.1
  * @since 2026-01-12
- * @lastModified 2026-01-12
+ * @lastModified 2026-01-15
  */
 
 import { Test, TestingModule } from '@nestjs/testing';
@@ -25,7 +26,6 @@ describe('RegisterService', () => {
   let service: RegisterService;
   let loginCoreService: jest.Mocked<LoginCoreService>;
   let zulipAccountService: jest.Mocked<ZulipAccountService>;
-  let apiKeySecurityService: jest.Mocked<ApiKeySecurityService>;
 
   const mockUser = {
     id: BigInt(1),
@@ -96,7 +96,6 @@ describe('RegisterService', () => {
     service = module.get<RegisterService>(RegisterService);
     loginCoreService = module.get(LoginCoreService);
     zulipAccountService = module.get(ZulipAccountService);
-    apiKeySecurityService = module.get(ApiKeySecurityService);
 
     // 设置默认的mock返回值
     const mockTokenPair = {