Run the following and it will show up as unknown:

var externalSvc = builder.AddExternalService("svc", builder.AddParameter("url"));

This is because it accesses the parameter's value directly https://github.com/dotnet/aspire/blob/da81496b05712535a784f5cf8218b627e958eef0/src/Aspire.Hosting/ExternalServiceBuilderExtensions.cs#L107

Which throws if it wasn't provided (or can even throw if there's an error getting the value). We should use IValueProvider.GetValueAsync and we should gracefully propagate errors from the parameter resolution to this resource's state.

cc @DamianEdwards