test: 为代码审核修复添加详细单元测试（7个测试文件，50+测试用例）

新增测试文件:
- cors_test.go: CORS 条件化头部测试（12个测试，覆盖白名单/黑名单/通配符/凭证/多源/Vary）
- gateway_helper_backoff_test.go: nextBackoff 退避测试（6个测试+基准，验证指数增长/边界/抖动/收敛）
- billing_cache_jitter_test.go: jitteredTTL 抖动测试（5个测试+基准，验证范围/上界/方差/均值）
- subscription_calculate_progress_test.go: calculateProgress 纯函数测试（9个测试，覆盖日/周/月限额/超限截断/过期）
- openai_gateway_handler_test.go: SSE JSON 转义测试（7个子用例，验证双引号/反斜杠/换行符安全）

更新测试文件:
- response_transformer_test.go: 增强 generateRandomID 测试（7个测试，含并发/字符集/降级计数器）
- security_headers_test.go: 适配 GenerateNonce 新签名
- api_key_auth_test.go: 适配 NewSubscriptionService 新参数

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/server/middleware/api_key_auth_test.go

@@ -60,7 +60,7 @@ func TestSimpleModeBypassesQuotaCheck(t *testing.T) {
 	t.Run("simple_mode_bypasses_quota_check", func(t *testing.T) {
 		cfg := &config.Config{RunMode: config.RunModeSimple}
 		apiKeyService := service.NewAPIKeyService(apiKeyRepo, nil, nil, nil, nil, nil, cfg)
-		subscriptionService := service.NewSubscriptionService(nil, &stubUserSubscriptionRepo{}, nil, cfg)
+		subscriptionService := service.NewSubscriptionService(nil, &stubUserSubscriptionRepo{}, nil, nil, cfg)
 		router := newAuthTestRouter(apiKeyService, subscriptionService, cfg)
 
 		w := httptest.NewRecorder()
@@ -99,7 +99,7 @@ func TestSimpleModeBypassesQuotaCheck(t *testing.T) {
 			resetWeekly:    func(ctx context.Context, id int64, start time.Time) error { return nil },
 			resetMonthly:   func(ctx context.Context, id int64, start time.Time) error { return nil },
 		}
-		subscriptionService := service.NewSubscriptionService(nil, subscriptionRepo, nil, cfg)
+		subscriptionService := service.NewSubscriptionService(nil, subscriptionRepo, nil, nil, cfg)
 		router := newAuthTestRouter(apiKeyService, subscriptionService, cfg)
 
 		w := httptest.NewRecorder()