fix(安全): 关闭白名单时保留最小校验与默认白名单

实现 allow_insecure_http 并在关闭校验时执行最小格式验证
- 关闭 allowlist 时要求 URL 可解析且 scheme 合规
- 响应头过滤关闭时使用默认白名单策略
- 更新相关文档、示例与测试覆盖

backend/internal/service/gateway_service.go

@@ -2206,7 +2206,11 @@ func (s *GatewayService) countTokensError(c *gin.Context, status int, errType, m
 
 func (s *GatewayService) validateUpstreamBaseURL(raw string) (string, error) {
 	if s.cfg != nil && !s.cfg.Security.URLAllowlist.Enabled {
-		return strings.TrimSpace(raw), nil
+		normalized, err := urlvalidator.ValidateURLFormat(raw, s.cfg.Security.URLAllowlist.AllowInsecureHTTP)
+		if err != nil {
+			return "", fmt.Errorf("invalid base_url: %w", err)
+		}
+		return normalized, nil
 	}
 	normalized, err := urlvalidator.ValidateHTTPSURL(raw, urlvalidator.ValidationOptions{
 		AllowedHosts:     s.cfg.Security.URLAllowlist.UpstreamHosts,