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/provider/client.go

@@ -50,6 +50,7 @@ type Client struct {
 }
 
 // ProviderClient 供应商客户端接口
+//go:generate go run go.uber.org/mock/mockgen -source=client.go -destination=../../tests/mocks/mock_provider_client.go -package=mocks
 type ProviderClient interface {
 	Send(ctx context.Context, spec conversion.HTTPRequestSpec) (*conversion.HTTPResponseSpec, error)
 	SendStream(ctx context.Context, spec conversion.HTTPRequestSpec) (<-chan StreamEvent, error)