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/helpers.go

@@ -7,49 +7,40 @@ import (
 
 	"nex/backend/internal/config"
 
-	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 	"gorm.io/driver/sqlite"
 	"gorm.io/gorm"
 )
 
-// SetupTestDB initializes an in-memory SQLite database with auto-migration.
-// Uses :memory: mode with MaxOpenConns(1) to ensure all operations share the
-// same connection, avoiding "database is closed" errors from connection pool.
-// Enables foreign key constraints for SQLite.
 func SetupTestDB(t *testing.T) *gorm.DB {
 	t.Helper()
 
 	db, err := gorm.Open(sqlite.Open(":memory:?_foreign_keys=on"), &gorm.Config{})
-	assert.NoError(t, err, "failed to open test database")
+	require.NoError(t, err, "failed to open test database")
 
-	// 限制为单连接，确保 :memory: 数据库不被连接池丢弃
 	sqlDB, err := db.DB()
-	assert.NoError(t, err, "failed to get underlying sql.DB")
+	require.NoError(t, err, "failed to get underlying sql.DB")
 	sqlDB.SetMaxOpenConns(1)
 	sqlDB.SetConnMaxLifetime(0)
 
 	err = db.AutoMigrate(&config.Provider{}, &config.Model{}, &config.UsageStats{})
-	assert.NoError(t, err, "failed to auto-migrate test database")
+	require.NoError(t, err, "failed to auto-migrate test database")
 
 	return db
 }
 
-// CleanupTestDB closes the database after a brief delay to allow async
-// goroutines (e.g. stats recording) to finish.
 func CleanupTestDB(t *testing.T, db *gorm.DB) {
 	t.Helper()
 
-	// 等待异步 goroutine（如 statsService.Record）完成
 	time.Sleep(50 * time.Millisecond)
 
 	sqlDB, err := db.DB()
-	assert.NoError(t, err, "failed to get underlying sql.DB")
+	require.NoError(t, err, "failed to get underlying sql.DB")
 
 	err = sqlDB.Close()
-	assert.NoError(t, err, "failed to close test database")
+	require.NoError(t, err, "failed to close test database")
 }
 
-// CreateTestProvider creates a test provider and returns it.
 func CreateTestProvider(t *testing.T, db *gorm.DB, id string) config.Provider {
 	t.Helper()
 
@@ -62,13 +53,11 @@ func CreateTestProvider(t *testing.T, db *gorm.DB, id string) config.Provider {
 	}
 
 	err := db.Create(&provider).Error
-	assert.NoError(t, err, "failed to create test provider")
+	require.NoError(t, err, "failed to create test provider")
 
 	return provider
 }
 
-// CreateTestModel creates a test model and returns it.
-// Does NOT assert on error - returns the model and error for caller to verify.
 func CreateTestModel(t *testing.T, db *gorm.DB, id string, providerID string, modelName string) (config.Model, error) {
 	t.Helper()