fix(openai): handle missing messages stream terminal

backend/internal/handler/openai_gateway_handler.go

@@ -758,6 +758,7 @@ func (h *OpenAIGatewayHandler) Messages(c *gin.Context) {
 		if channelMappingMsg.Mapped {
 			forwardBody = h.gatewayService.ReplaceModelInBody(body, channelMappingMsg.MappedModel)
 		}
+		writerSizeBeforeForward := c.Writer.Size()
 		result, err := func() (*service.OpenAIForwardResult, error) {
 			defer func() {
 				if accountReleaseFunc != nil {
@@ -787,6 +788,10 @@ func (h *OpenAIGatewayHandler) Messages(c *gin.Context) {
 			} else {
 				var failoverErr *service.UpstreamFailoverError
 				if errors.As(err, &failoverErr) {
+					if c.Writer.Size() != writerSizeBeforeForward {
+						h.handleAnthropicFailoverExhausted(c, failoverErr, true)
+						return
+					}
 					h.gatewayService.ReportOpenAIAccountScheduleResult(account.ID, false, nil)
 					// 池模式：同账号重试
 					if failoverErr.RetryableOnSameAccount {
@@ -827,6 +832,13 @@ func (h *OpenAIGatewayHandler) Messages(c *gin.Context) {
 					)
 					continue
 				}
+				if result != nil && result.ClientDisconnect {
+					reqLog.Info("openai_messages.client_disconnected",
+						zap.Int64("account_id", account.ID),
+						zap.Error(err),
+					)
+					return
+				}
 				h.gatewayService.ReportOpenAIAccountScheduleResult(account.ID, false, nil)
 				wroteFallback := h.ensureAnthropicErrorResponse(c, streamStarted)
 				reqLog.Warn("openai_messages.forward_failed",