CheckIPAddressAvailability does not detect ILB ASE addresses

https://docs.microsoft.com/en-us/rest/api/virtualnetwork/virtualnetworks/checkipaddressavailability returns
```
{
"available": true,
"isPlatformReserved": false
}
```
when calling with an IP adress that is in use by an ILB ASE.

The issue names the CheckIPAddressAvailability REST endpoint but no repository file or test. Start by reviewing the linked Virtual Networks API specification and the reported ILB ASE response, then determine where this behavior is defined or tracked. Done means the endpoint no longer reports an in-use ILB ASE address as available, with coverage for that case.

Written by the indexing model from the issue text.