Fix inconsistent URLTest results

protocol/vless/outbound.go

@@ -27,6 +27,8 @@ func RegisterOutbound(registry *outbound.Registry) {
 	outbound.Register[option.VLESSOutboundOptions](registry, C.TypeVLESS, NewOutbound)
 }
 
+var _ adapter.OutboundWithMultiplex = (*Outbound)(nil)
+
 type Outbound struct {
 	outbound.Adapter
 	logger          logger.ContextLogger
@@ -127,6 +129,10 @@ func (h *Outbound) ListenPacket(ctx context.Context, destination M.Socksaddr) (n
 	}
 }
 
+func (h *Outbound) MultiplexEnabled() bool {
+	return h.multiplexDialer != nil
+}
+
 func (h *Outbound) InterfaceUpdated() {
 	if h.transport != nil {
 		h.transport.Close()