feat: 系统性改进后端测试体系

- 新增 6 个测试场景 (config load pipe, handler errors, service aggregation, engine degradation, openai decoder edges, negative tests)
- 更新测试工具规格 (mockgen, in-memory SQLite)
- 覆盖率目标从 >80% 提升至 >85%
- 新增 test-unit 和 test-integration Makefile 命令
- 新增死代码清理和 mockgen 需求
- 归档变更至 openspec/changes/archive/2026-04-22-improve-backend-testing/

backend/internal/service/model_service.go

@@ -2,6 +2,8 @@ package service
 
 import "nex/backend/internal/domain"
 
+//go:generate go run go.uber.org/mock/mockgen -source=model_service.go -destination=../../tests/mocks/mock_model_service.go -package=mocks
+
 // ModelService 模型服务接口
 type ModelService interface {
 	Create(model *domain.Model) error