Support VPN client gateway selection in path calculation for firewall-based VPN termination

NetBrain path trace currently does not allow a VPN-terminating device to be selected as the first-hop gateway when calculating the path for a VPN client, even when that device is the actual point of entry into the network. In environments where multiple gateway-capable devices exist, the path engine may default to an unrelated device, producing an inaccurate path that does not reflect real traffic flow. This enhancement would allow the path trace engine to correctly identify and offer the actual VPN-terminating device as a selectable gateway option for VPN client path calculations, ensuring the resulting path reflects the true routing behavior. Customers relying on path trace for VPN troubleshooting would gain accurate, end-to-end visibility without needing to manually correct or reinterpret an incorrect calculated path.