Allow Custom DHCP Range for Isolated Guest Networks

Currently, in isolated guest networks (with Source NAT), DHCP ranges are automatically derived from the network CIDR and cannot be explicitly defined.

Currently, users rely on CIDR adjustments by editing the network after it is created. It would be beneficial to allow administrators to directly specify the desired DHCP range during network creation for better flexibility and usability.

This feature would allow administrators to explicitly specify a DHCP range (e.g., 192.168.100.10 - 192.168.100.200) at the time of isolated guest network creation.

Start by tracing isolated guest network creation and where DHCP ranges are derived from the network CIDR. Define how an explicitly supplied range such as 192.168.100.10–192.168.100.200 is accepted during creation, then verify that the requested range is retained and used for DHCP configuration.

Written by the indexing model from the issue text.