AKS: Support AsExisting() for pre-provisioned clusters

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

## Expected
```csharp
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)