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);
}

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[marcpopMSFT]

I was unable to test this to recreate the exact situation, but the change looks good to me.

[Copilot]

Pull request overview

Improves diagnosability of workload advertising-manifest update failures by surfacing the specific failing NuGet source when protocol-level errors occur during package metadata queries.

Changes:

• Wrapes non-ignored FatalProtocolException in GetPackageMetadataAsync(...) with a NuGetPackageInstallerException that includes the failing source URL in the message.
• Adds a unit test asserting the wrapped exception message includes the source and preserves the FatalProtocolException as InnerException.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/Cli/dotnet/NugetPackageDownloader/NuGetPackageDownloader.cs	Wraps FatalProtocolException to include source context in surfaced errors.
test/Microsoft.DotNet.PackageInstall.Tests/NuGetPackageInstallerTests.cs	Adds test validating source-aware error messaging for failed protocol sources.