[FEATURE REQ] Support private IP prefix (Prefix on NIC) on VMSS IP configurations

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/main/specification/compute/resource-manager/Microsoft.Compute/Compute/Compute/models.tsp

### API Spec version

2026-03-01

### Please describe the feature.

### Summary

`Microsoft.Network` supports "Prefix on NIC": assigning a private IP **CIDR block** to a network interface IP configuration instead of a single address. `Microsoft.Compute` exposes no equivalent for VMSS instance NICs, which are defined inline via the VMSS network profile and cannot be configured through `Microsoft.Network` out-of-band.

### Where it exists today (Network)

`Microsoft.Network` defines this on the NIC IP configuration:

- File: `specification/network/resource-manager/Microsoft.Network/Network/Common/main.tsp`
- Model: `InterfaceIPConfigurationPropertiesFormat`
- Field:

```tsp
/**
* The private IP address prefix length. If specified and the allocation method is
* dynamic, the service will allocate a CIDR block instead of a single IP address.
*/
@minValue(1)
@maxValue(128)
privateIPAddressPrefixLength?: int32 | null;
```

### What's missing (Compute / VMSS)

The corresponding VMSS IP-configuration model has no prefix-length property:

- File: `specification/compute/resource-manager/Microsoft.Compute/Compute/Compute/models.tsp`
- Model: `VirtualMachineScaleSetIPConfigurationProperties` (and the update variant used for PATCH)
- Current fields: `subnet`, `primary`, `publicIPAddressConfiguration`, `privateIPAddressVersion`, `applicationGatewayBackendAddressPools`, `applicationSecurityGroups`, `loadBalancerBackendAddressPools`, `loadBalancerInboundNatPools`, no `privateIPAddressPrefixLength`.

### Requested change

Add a `privateIPAddressPrefixLength?: int32` property to `VirtualMachineScaleSetIPConfigurationProperties` (and its update counterpart), mirroring the `Microsoft.Network` semantics, so a VMSS network profile can request a CIDR block per instance NIC.

### Motivation

Workloads that need a routable IP block per instance (e.g. per-pod/container IP allocation, NVA scenarios) can already get this on standalone NICs and individual VMs, but not on scale-set instances. This blocks the pattern on VMSS entirely from the typed SDKs (e.g. `armcompute` for Go has no field to set), forcing raw-REST/template workarounds.

Start in specification/compute/resource-manager/Microsoft.Compute/Compute/Compute/models.tsp and inspect VirtualMachineScaleSetIPConfigurationProperties plus its update counterpart. Compare the existing privateIPAddressPrefixLength definition in specification/network/resource-manager/Microsoft.Network/Network/Common/main.tsp, then verify both VMSS models expose the requested property with matching semantics.

Written by the indexing model from the issue text.