test(openai): 修复回归测试未使用字段告警

移除订阅扣费 stub 中未被使用的状态字段与赋值，
消除 golangci-lint 的 unused 告警，保持回归测试语义不变。
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/openai_gateway_record_usage_test.go

@@ -44,12 +44,10 @@ type openAIRecordUsageSubRepoStub struct {
 
 	incrementCalls int
 	incrementErr   error
-	lastAmount     float64
 }
 
 func (s *openAIRecordUsageSubRepoStub) IncrementUsage(ctx context.Context, id int64, costUSD float64) error {
 	s.incrementCalls++
-	s.lastAmount = costUSD
 	return s.incrementErr
 }