Include failing NuGet source in workload advertising manifest update errors

[Copilot]

Workload advertising manifest updates on secured/inaccessible feeds surfaced repeated 401 Unauthorized failures without identifying which feed failed, making diagnosis difficult. This change propagates source context into the surfaced error so users can immediately identify the problematic feed.

• Error propagation: include source on protocol failures

  • Updated NuGetPackageDownloader.GetPackageMetadataAsync(...) to wrap non-ignored FatalProtocolException with NuGetPackageInstallerException that includes the failing source URL and original protocol message.
  • This improves workload advertising-manifest failure output because WorkloadManifestUpdater already reports the thrown exception message.

• Targeted coverage for source-aware failures

  • Added GivenAFailedSourceItShouldIncludeSourceInError in NuGetPackageInstallerTests.
  • Validates the wrapped exception message includes source context, preserves FatalProtocolException as InnerException, and carries through the inner message.

catch (FatalProtocolException e)
{
    throw new NuGetPackageInstallerException(
        $"{string.Format(CliStrings.FailedToLoadNuGetSource, source.Source)}: {e.Message}", e);
}