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/repository/provider_repo_impl.go

@@ -71,25 +71,6 @@ func (r *providerRepository) Delete(id string) error {
 	return nil
 }
 
-// ListEnabledModels 返回所有启用的模型（关联启用的供应商）
-func (r *providerRepository) ListEnabledModels() ([]domain.Model, error) {
-	var models []domain.Model
-	err := r.db.Joins("JOIN providers ON providers.id = models.provider_id").
-		Where("models.enabled = ? AND providers.enabled = ?", true, true).
-		Find(&models).Error
-	return models, err
-}
-
-// FindByProviderAndModelName 按 provider_id 和 model_name 查询模型
-func (r *providerRepository) FindByProviderAndModelName(providerID, modelName string) (*domain.Model, error) {
-	var model domain.Model
-	err := r.db.Where("provider_id = ? AND model_name = ?", providerID, modelName).First(&model).Error
-	if err != nil {
-		return nil, err
-	}
-	return &model, nil
-}
-
 func toDomainProvider(p *config.Provider) domain.Provider {
 	return domain.Provider{
 		ID:        p.ID,