AKS: Support AsExisting() for pre-provisioned clusters

[mitchdenny]

Problem

AsExisting() is not implemented for AzureKubernetesEnvironmentResource. Users cannot reference a pre-provisioned AKS cluster — they must always provision a new one.

Expected

var nameParam = builder.AddParameter("aks-name");
var rgParam = builder.AddParameter("aks-rg");
var aks = builder.AddAzureKubernetesEnvironment("aks")
    .AsExisting(nameParam, rgParam);

Should use the established ExistingAzureResourceAnnotation pattern and skip Bicep generation for the cluster itself while still supporting workload identity, Helm deployment, etc.

Related

Part of AKS hosting support (PR #16088)