AKS vnetSubnetID not filled for agentPoolProfiles

## Problem
Endpoint returns for my newer AKS clusters no vnetSubnetID in agentPoolProfiles
## Endpoint
https://management.azure.com:443 "GET /subscriptions/{subscription}/providers/Microsoft.ContainerService/managedClusters?api-version=2019-10-01 HTTP/1.1"
## Response example of a newer cluster
```
"agentPoolProfiles": [
{
"availabilityZones": null,
"count": 3,
"enableAutoScaling": false,
"enableNodePublicIp": false,
"maxCount": null,
"maxPods": 110,
"minCount": null,
"name": "nodepool1",
"nodeTaints": null,
"orchestratorVersion": "1.15.7",
"osDiskSizeGb": 100,
"osType": "Linux",
"provisioningState": "Succeeded",
"scaleSetEvictionPolicy": null,
"scaleSetPriority": null,
"type": "VirtualMachineScaleSets",
"vmSize": "Standard_DS2_v2"
}
],
```
## Response example of an older cluster (probably created via Azure Portal)
```
"agentPoolProfiles": [
{
"availabilityZones": null,
"count": 3,
"enableAutoScaling": null,
"enableNodePublicIp": false,
"maxCount": null,
"maxPods": 30,
"minCount": null,
"name": "agentpool",
"nodeTaints": null,
"orchestratorVersion": "1.15.7",
"osDiskSizeGb": 100,
"osType": "Linux",
"provisioningState": "Succeeded",
"scaleSetEvictionPolicy": null,
"scaleSetPriority": null,
"type": "AvailabilitySet",
"vmSize": "Standard_DS2_v2",
"vnetSubnetId": "/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupId}/providers/Microsoft.Network/virtualNetworks/{vnetid}/subnets/default"
}
],
```

Start by locating the managedClusters API specification for api-version 2019-10-01 and inspect the agentPoolProfiles schema against the two response examples. Confirm whether vnetSubnetId is missing or incorrectly modeled for newer AKS clusters, then update the specification and its validation coverage so the documented response matches the service behavior.

Written by the indexing model from the issue text.