fix: 修复函数签名变更后的调用参数不匹配

- handleUpstreamError 补齐新增的三个参数 (0, "", false)
- handleStreamingResponse 移除已删除的 nil 参数

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

backend/internal/service/gateway_service_streaming_test.go

@@ -42,7 +42,7 @@ func TestGatewayService_StreamingReusesScannerBufferAndStillParsesUsage(t *testi
 		_, _ = pw.Write([]byte("data: [DONE]\n\n"))
 	}()
 
-	result, err := svc.handleStreamingResponse(context.Background(), resp, c, &Account{ID: 1}, time.Now(), "model", "model", nil, false)
+	result, err := svc.handleStreamingResponse(context.Background(), resp, c, &Account{ID: 1}, time.Now(), "model", "model", false)
 	_ = pr.Close()
 	require.NoError(t, err)
 	require.NotNil(t, result)