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_stats_service.go

@@ -0,0 +1,85 @@
+// Code generated by MockGen. DO NOT EDIT.
+// Source: stats_service.go
+//
+// Generated by this command:
+//
+//	mockgen -source=stats_service.go -destination=../../tests/mocks/mock_stats_service.go -package=mocks
+//
+
+// Package mocks is a generated GoMock package.
+package mocks
+
+import (
+	domain "nex/backend/internal/domain"
+	reflect "reflect"
+	time "time"
+
+	gomock "go.uber.org/mock/gomock"
+)
+
+// MockStatsService is a mock of StatsService interface.
+type MockStatsService struct {
+	ctrl     *gomock.Controller
+	recorder *MockStatsServiceMockRecorder
+	isgomock struct{}
+}
+
+// MockStatsServiceMockRecorder is the mock recorder for MockStatsService.
+type MockStatsServiceMockRecorder struct {
+	mock *MockStatsService
+}
+
+// NewMockStatsService creates a new mock instance.
+func NewMockStatsService(ctrl *gomock.Controller) *MockStatsService {
+	mock := &MockStatsService{ctrl: ctrl}
+	mock.recorder = &MockStatsServiceMockRecorder{mock}
+	return mock
+}
+
+// EXPECT returns an object that allows the caller to indicate expected use.
+func (m *MockStatsService) EXPECT() *MockStatsServiceMockRecorder {
+	return m.recorder
+}
+
+// Aggregate mocks base method.
+func (m *MockStatsService) Aggregate(stats []domain.UsageStats, groupBy string) []map[string]any {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "Aggregate", stats, groupBy)
+	ret0, _ := ret[0].([]map[string]any)
+	return ret0
+}
+
+// Aggregate indicates an expected call of Aggregate.
+func (mr *MockStatsServiceMockRecorder) Aggregate(stats, groupBy any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "Aggregate", reflect.TypeOf((*MockStatsService)(nil).Aggregate), stats, groupBy)
+}
+
+// Get mocks base method.
+func (m *MockStatsService) Get(providerID, modelName string, startDate, endDate *time.Time) ([]domain.UsageStats, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "Get", providerID, modelName, startDate, endDate)
+	ret0, _ := ret[0].([]domain.UsageStats)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// Get indicates an expected call of Get.
+func (mr *MockStatsServiceMockRecorder) Get(providerID, modelName, startDate, endDate any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "Get", reflect.TypeOf((*MockStatsService)(nil).Get), providerID, modelName, startDate, endDate)
+}
+
+// Record mocks base method.
+func (m *MockStatsService) Record(providerID, modelName string) error {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "Record", providerID, modelName)
+	ret0, _ := ret[0].(error)
+	return ret0
+}
+
+// Record indicates an expected call of Record.
+func (mr *MockStatsServiceMockRecorder) Record(providerID, modelName any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "Record", reflect.TypeOf((*MockStatsService)(nil).Record), providerID, modelName)
+}