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.

首先跟踪隔离来宾网络的创建过程，以及 DHCP 范围从网络 CIDR 派生的位置。明确在创建过程中如何接受显式提供的范围（例如 192.168.100.10–192.168.100.200），然后验证所请求的范围会被保留并用于 DHCP 配置。

由索引模型根据 Issue 内容生成。