Add Integration Tests for API Endpoints

[FerdinandSu]

Description / 描述

English:
Create integration tests for all API endpoints using WebApplicationFactory:

• Test all endpoints in LedgerController
• Test all endpoints in ConfigController
• Test authentication/authorization flows
• Test error responses
• Test with different data scenarios

中文:
使用 WebApplicationFactory 为所有 API 端点创建集成测试：

• 测试 LedgerController 中的所有端点
• 测试 ConfigController 中的所有端点
• 测试身份验证/授权流程
• 测试错误响应
• 使用不同的数据场景进行测试

Difficulty / 难度

Medium-Hard / 中等-困难

Estimated Time / 预计时间

8-10 hours

Acceptance Criteria / 验收标准

• Create test project: tests/WebLedger.IntegrationTests
• Use WebApplicationFactory for testing
• Write tests for all API endpoints
• Mock external dependencies
• Add authentication tests
• Integrate with CI/CD pipeline

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