fix: 修复模型不存在时的返回值，添加错误处理逻辑

main.go

@@ -348,7 +348,7 @@ func getEndpointID(openaiModel string) string {
 	if endpoint, exists := modelMap[model]; exists {
 		return endpoint
 	}
-	return defaultModel
+	return ""
 }
 
 // 创建会话
@@ -456,6 +456,17 @@ func chatCompletions(c *gin.Context) {
 	}
 
 	endpointID := getEndpointID(req.Model)
+	if endpointID == "" {
+		c.JSON(http.StatusBadRequest, gin.H{
+			"error": map[string]interface{}{
+				"message": fmt.Sprintf("The model '%s' does not exist", req.Model),
+				"type":    "invalid_request_error",
+				"param":   "model",
+				"code":    "model_not_found",
+			},
+		})
+		return
+	}
 
 	// 添加模型和端点的日志记录
 	log.Printf("【模型请求】模型: %s, 端点: %s, 流式: %t", req.Model, endpointID, req.Stream)