feat(backend): 提交后端审计修复与配套测试改动

backend/internal/handler/openai_gateway_handler.go

@@ -365,8 +365,12 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 				)
 				continue
 			}
-			// Error response already handled in Forward, just log
-			reqLog.Error("openai.forward_failed", zap.Int64("account_id", account.ID), zap.Error(err))
+			wroteFallback := h.ensureForwardErrorResponse(c, streamStarted)
+			reqLog.Error("openai.forward_failed",
+				zap.Int64("account_id", account.ID),
+				zap.Bool("fallback_error_response_written", wroteFallback),
+				zap.Error(err),
+			)
 			return
 		}
 
@@ -521,6 +525,15 @@ func (h *OpenAIGatewayHandler) handleStreamingAwareError(c *gin.Context, status
 	h.errorResponse(c, status, errType, message)
 }
 
+// ensureForwardErrorResponse 在 Forward 返回错误但尚未写响应时补写统一错误响应。
+func (h *OpenAIGatewayHandler) ensureForwardErrorResponse(c *gin.Context, streamStarted bool) bool {
+	if c == nil || c.Writer == nil || c.Writer.Written() {
+		return false
+	}
+	h.handleStreamingAwareError(c, http.StatusBadGateway, "upstream_error", "Upstream request failed", streamStarted)
+	return true
+}
+
 // errorResponse returns OpenAI API format error response
 func (h *OpenAIGatewayHandler) errorResponse(c *gin.Context, status int, errType, message string) {
 	c.JSON(status, gin.H{