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/tests/mocks/mock_routing_service.go

@@ -0,0 +1,56 @@
+// Code generated by MockGen. DO NOT EDIT.
+// Source: routing_service.go
+//
+// Generated by this command:
+//
+//	mockgen -source=routing_service.go -destination=../../tests/mocks/mock_routing_service.go -package=mocks
+//
+
+// Package mocks is a generated GoMock package.
+package mocks
+
+import (
+	domain "nex/backend/internal/domain"
+	reflect "reflect"
+
+	gomock "go.uber.org/mock/gomock"
+)
+
+// MockRoutingService is a mock of RoutingService interface.
+type MockRoutingService struct {
+	ctrl     *gomock.Controller
+	recorder *MockRoutingServiceMockRecorder
+	isgomock struct{}
+}
+
+// MockRoutingServiceMockRecorder is the mock recorder for MockRoutingService.
+type MockRoutingServiceMockRecorder struct {
+	mock *MockRoutingService
+}
+
+// NewMockRoutingService creates a new mock instance.
+func NewMockRoutingService(ctrl *gomock.Controller) *MockRoutingService {
+	mock := &MockRoutingService{ctrl: ctrl}
+	mock.recorder = &MockRoutingServiceMockRecorder{mock}
+	return mock
+}
+
+// EXPECT returns an object that allows the caller to indicate expected use.
+func (m *MockRoutingService) EXPECT() *MockRoutingServiceMockRecorder {
+	return m.recorder
+}
+
+// RouteByModelName mocks base method.
+func (m *MockRoutingService) RouteByModelName(providerID, modelName string) (*domain.RouteResult, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "RouteByModelName", providerID, modelName)
+	ret0, _ := ret[0].(*domain.RouteResult)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// RouteByModelName indicates an expected call of RouteByModelName.
+func (mr *MockRoutingServiceMockRecorder) RouteByModelName(providerID, modelName any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "RouteByModelName", reflect.TypeOf((*MockRoutingService)(nil).RouteByModelName), providerID, modelName)
+}