fix(billing): 修复客户端取消请求时计费丢失问题

检测 context.Canceled 作为客户端断开信号，返回已收集的 usage 而非错误

backend/internal/service/gateway_service.go

@@ -109,12 +109,13 @@ type ClaudeUsage struct {
 
 // ForwardResult 转发结果
 type ForwardResult struct {
-	RequestID    string
-	Usage        ClaudeUsage
-	Model        string
-	Stream       bool
-	Duration     time.Duration
-	FirstTokenMs *int // 首字时间（流式请求）
+	RequestID        string
+	Usage            ClaudeUsage
+	Model            string
+	Stream           bool
+	Duration         time.Duration
+	FirstTokenMs     *int // 首字时间（流式请求）
+	ClientDisconnect bool // 客户端是否在流式传输过程中断开
 
 	// 图片生成计费字段（仅 gemini-3-pro-image 使用）
 	ImageCount int    // 生成的图片数量
@@ -1465,6 +1466,7 @@ func (s *GatewayService) Forward(ctx context.Context, c *gin.Context, account *A
 	// 处理正常响应
 	var usage *ClaudeUsage
 	var firstTokenMs *int
+	var clientDisconnect bool
 	if reqStream {
 		streamResult, err := s.handleStreamingResponse(ctx, resp, c, account, startTime, originalModel, reqModel)
 		if err != nil {
@@ -1477,6 +1479,7 @@ func (s *GatewayService) Forward(ctx context.Context, c *gin.Context, account *A
 		}
 		usage = streamResult.usage
 		firstTokenMs = streamResult.firstTokenMs
+		clientDisconnect = streamResult.clientDisconnect
 	} else {
 		usage, err = s.handleNonStreamingResponse(ctx, resp, c, account, originalModel, reqModel)
 		if err != nil {
@@ -1485,12 +1488,13 @@ func (s *GatewayService) Forward(ctx context.Context, c *gin.Context, account *A
 	}
 
 	return &ForwardResult{
-		RequestID:    resp.Header.Get("x-request-id"),
-		Usage:        *usage,
-		Model:        originalModel, // 使用原始模型用于计费和日志
-		Stream:       reqStream,
-		Duration:     time.Since(startTime),
-		FirstTokenMs: firstTokenMs,
+		RequestID:        resp.Header.Get("x-request-id"),
+		Usage:            *usage,
+		Model:            originalModel, // 使用原始模型用于计费和日志
+		Stream:           reqStream,
+		Duration:         time.Since(startTime),
+		FirstTokenMs:     firstTokenMs,
+		ClientDisconnect: clientDisconnect,
 	}, nil
 }
 
@@ -1845,8 +1849,9 @@ func (s *GatewayService) handleRetryExhaustedError(ctx context.Context, resp *ht
 
 // streamingResult 流式响应结果
 type streamingResult struct {
-	usage        *ClaudeUsage
-	firstTokenMs *int
+	usage            *ClaudeUsage
+	firstTokenMs     *int
+	clientDisconnect bool // 客户端是否在流式传输过程中断开
 }
 
 func (s *GatewayService) handleStreamingResponse(ctx context.Context, resp *http.Response, c *gin.Context, account *Account, startTime time.Time, originalModel, mappedModel string) (*streamingResult, error) {
@@ -1942,14 +1947,27 @@ func (s *GatewayService) handleStreamingResponse(ctx context.Context, resp *http
 	}
 
 	needModelReplace := originalModel != mappedModel
+	clientDisconnected := false // 客户端断开标志，断开后继续读取上游以获取完整usage
 
 	for {
 		select {
 		case ev, ok := <-events:
 			if !ok {
-				return &streamingResult{usage: usage, firstTokenMs: firstTokenMs}, nil
+				// 上游完成，返回结果
+				return &streamingResult{usage: usage, firstTokenMs: firstTokenMs, clientDisconnect: clientDisconnected}, nil
 			}
 			if ev.err != nil {
+				// 检测 context 取消（客户端断开会导致 context 取消，进而影响上游读取）
+				if errors.Is(ev.err, context.Canceled) || errors.Is(ev.err, context.DeadlineExceeded) {
+					log.Printf("Context canceled during streaming, returning collected usage")
+					return &streamingResult{usage: usage, firstTokenMs: firstTokenMs, clientDisconnect: true}, nil
+				}
+				// 客户端已通过写入失败检测到断开，上游也出错了，返回已收集的 usage
+				if clientDisconnected {
+					log.Printf("Upstream read error after client disconnect: %v, returning collected usage", ev.err)
+					return &streamingResult{usage: usage, firstTokenMs: firstTokenMs, clientDisconnect: true}, nil
+				}
+				// 客户端未断开，正常的错误处理
 				if errors.Is(ev.err, bufio.ErrTooLong) {
 					log.Printf("SSE line too long: account=%d max_size=%d error=%v", account.ID, maxLineSize, ev.err)
 					sendErrorEvent("response_too_large")
@@ -1960,38 +1978,40 @@ func (s *GatewayService) handleStreamingResponse(ctx context.Context, resp *http
 			}
 			line := ev.line
 			if line == "event: error" {
+				// 上游返回错误事件，如果客户端已断开仍返回已收集的 usage
+				if clientDisconnected {
+					return &streamingResult{usage: usage, firstTokenMs: firstTokenMs, clientDisconnect: true}, nil
+				}
 				return nil, errors.New("have error in stream")
 			}
 
 			// Extract data from SSE line (supports both "data: " and "data:" formats)
+			var data string
 			if sseDataRe.MatchString(line) {
-				data := sseDataRe.ReplaceAllString(line, "")
-
+				data = sseDataRe.ReplaceAllString(line, "")
 				// 如果有模型映射，替换响应中的model字段
 				if needModelReplace {
 					line = s.replaceModelInSSELine(line, mappedModel, originalModel)
 				}
+			}
 
-				// 转发行
+			// 写入客户端（统一处理 data 行和非 data 行）
+			if !clientDisconnected {
 				if _, err := fmt.Fprintf(w, "%s\n", line); err != nil {
-					sendErrorEvent("write_failed")
-					return &streamingResult{usage: usage, firstTokenMs: firstTokenMs}, err
+					clientDisconnected = true
+					log.Printf("Client disconnected during streaming, continuing to drain upstream for billing")
+				} else {
+					flusher.Flush()
 				}
-				flusher.Flush()
+			}
 
-				// 记录首字时间：第一个有效的 content_block_delta 或 message_start
-				if firstTokenMs == nil && data != "" && data != "[DONE]" {
+			// 无论客户端是否断开，都解析 usage（仅对 data 行）
+			if data != "" {
+				if firstTokenMs == nil && data != "[DONE]" {
 					ms := int(time.Since(startTime).Milliseconds())
 					firstTokenMs = &ms
 				}
 				s.parseSSEUsage(data, usage)
-			} else {
-				// 非 data 行直接转发
-				if _, err := fmt.Fprintf(w, "%s\n", line); err != nil {
-					sendErrorEvent("write_failed")
-					return &streamingResult{usage: usage, firstTokenMs: firstTokenMs}, err
-				}
-				flusher.Flush()
 			}
 
 		case <-intervalCh:
@@ -1999,6 +2019,11 @@ func (s *GatewayService) handleStreamingResponse(ctx context.Context, resp *http
 			if time.Since(lastRead) < streamInterval {
 				continue
 			}
+			if clientDisconnected {
+				// 客户端已断开，上游也超时了，返回已收集的 usage
+				log.Printf("Upstream timeout after client disconnect, returning collected usage")
+				return &streamingResult{usage: usage, firstTokenMs: firstTokenMs, clientDisconnect: true}, nil
+			}
 			log.Printf("Stream data interval timeout: account=%d model=%s interval=%s", account.ID, originalModel, streamInterval)
 			sendErrorEvent("stream_timeout")
 			return &streamingResult{usage: usage, firstTokenMs: firstTokenMs}, fmt.Errorf("stream data interval timeout")