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_repository.go

@@ -0,0 +1,71 @@
+// Code generated by MockGen. DO NOT EDIT.
+// Source: stats_repo.go
+//
+// Generated by this command:
+//
+//	mockgen -source=stats_repo.go -destination=../../tests/mocks/mock_stats_repository.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"
+)
+
+// MockStatsRepository is a mock of StatsRepository interface.
+type MockStatsRepository struct {
+	ctrl     *gomock.Controller
+	recorder *MockStatsRepositoryMockRecorder
+	isgomock struct{}
+}
+
+// MockStatsRepositoryMockRecorder is the mock recorder for MockStatsRepository.
+type MockStatsRepositoryMockRecorder struct {
+	mock *MockStatsRepository
+}
+
+// NewMockStatsRepository creates a new mock instance.
+func NewMockStatsRepository(ctrl *gomock.Controller) *MockStatsRepository {
+	mock := &MockStatsRepository{ctrl: ctrl}
+	mock.recorder = &MockStatsRepositoryMockRecorder{mock}
+	return mock
+}
+
+// EXPECT returns an object that allows the caller to indicate expected use.
+func (m *MockStatsRepository) EXPECT() *MockStatsRepositoryMockRecorder {
+	return m.recorder
+}
+
+// Query mocks base method.
+func (m *MockStatsRepository) Query(providerID, modelName string, startDate, endDate *time.Time) ([]domain.UsageStats, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "Query", providerID, modelName, startDate, endDate)
+	ret0, _ := ret[0].([]domain.UsageStats)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// Query indicates an expected call of Query.
+func (mr *MockStatsRepositoryMockRecorder) Query(providerID, modelName, startDate, endDate any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "Query", reflect.TypeOf((*MockStatsRepository)(nil).Query), providerID, modelName, startDate, endDate)
+}
+
+// Record mocks base method.
+func (m *MockStatsRepository) 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 *MockStatsRepositoryMockRecorder) Record(providerID, modelName any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "Record", reflect.TypeOf((*MockStatsRepository)(nil).Record), providerID, modelName)
+}