Get-AzVirtualNetworkUsageList should return the actual current value rather than 0 for delegated subnets

### Description

> ### `Get-AzVirtualNetworkUsageList` returns CurrentValue as 0 when subnet is delegated

**Describe the bug**
It should return the actual value rather than 0, we cant clearly see how many ips are used by the resource when the subnet is delegated.

**To Reproduce**
run Get-AzVirtualNetworkUsageList -ResourceGroupName "xyz-rg" -Name "xyz-vnet"

**Expected behavior**
It should return the actual value in the CurrentValue property to help us identity how many resources can be deployed of similar types when the subnet is delegated

### Script or Debug output

```PowerShell
Get-AzVirtualNetworkUsageList
```

### Environment data

_No response_

### Module versions

_No response_

### Error output

```PowerShell
returns currentvalue of 0
```

Start by running Get-AzVirtualNetworkUsageList -ResourceGroupName "xyz-rg" -Name "xyz-vnet" against a delegated subnet and inspect how CurrentValue is obtained. Compare the reported 0 with the actual resources or IP usage; done means the cmdlet returns the current delegated-subnet value instead of 0.

Written by the indexing model from the issue text.