Remove DefaultContainerRegistry from model when Foundry project uses explicit registry

[Copilot]

Description

Foundry projects create a default ACR but don't remove it when an explicit registry is specified via WithContainerRegistry, resulting in two ACRs being provisioned instead of one.

Applies the same pattern already used by AKS (AzureKubernetesEnvironmentExtensions.WithContainerRegistry): remove the default registry from the model and null the property when an explicit registry is set.

var acr = builder.AddAzureContainerRegistry("acr");
var foundry = builder.AddFoundry("foundry");
foundry.AddProject("my-project")
    .WithContainerRegistry(acr); // now correctly removes the default ACR

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • No
• Did you add public API?
  • Yes
  • No
• Does the change make any security assumptions or guarantees?
  • Yes
  • No
• Does the change require an update in our Aspire docs?
  • Yes
  • No

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• 7k6vsblobprodcus337.vsblob.vsassets.io
  • Triggering command: /usr/share/dotnet/dotnet dotnet build src/Aspire.Hosting.Foundry/Aspire.Hosting.Foundry.csproj (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)