Changing network address fails with "Network address space cannot fit all devices" even when possible

[jakub-tldr]

Description

Attempting to change the address of a WireGuard network results in error even though the new address is the same prefix length as the previous one and should accommodate the same number of devices.

Screen.Recording.2026-03-03.at.20.40.47.mov

Steps to Reproduce

1. Create a WireGuard network with address 10.1.1.0/29 (8 addresses, 5 usable
   host slots)
2. Connect a gateway to the network
3. Create a user and add 5 devices to them — the network is now completely
   full
4. Try to change the network address to another /29, e.g. 12.1.1.0/29

Cause (?)

let mut allowed_devices = location.get_allowed_devices(&mut *conn).await?;  // 5 user devices
let network_devices =
    Device::find_by_type_and_network(&mut *conn, DeviceType::Network, location.id).await?;
allowed_devices.extend(network_devices);  // + 1 gateway device = 6

let count = allowed_devices.len() + 3;   // 6 + 3 = 9
location.validate_network_size(count)?;  // 9 > 8 (size of /29) -> error

[moubctez]

Can't reproduce the issue.