[tests] Use dotnet-public for CI package resolution

[jonathanpeppers]

---

CFSClean reports network-isolation warnings when tests and CI tooling resolve packages directly from public NuGet endpoints. Route those package downloads through the existing dnceng dotnet-public Azure Artifacts feed and rely on the repository NuGet.config for generated test projects and the API-docs tool installation.

Direct .nupkg test downloads now use the feed's V3 package base address, while obsolete ExtraNuGetConfigSources public overrides are removed. No fallback to nuget.org is retained.

Xamarin.Google.Android.InstallReferrer 1.1.2.6 is not currently mirrored to dotnet-public; InstallAndRunTests.GradleFBProj will require that package to be mirrored.

• Useful description of why the change is necessary.
• Links to issues fixed
• Unit tests

Validation:

• FilterAssembliesTests: 4 passed
• Forced no-cache restore of the generated DotNetBuild project using its generated NuGet.config
• boots 1.1.0.36 installation using the repository NuGet.config
• Test and automation endpoint scan for api.nuget.org, www.nuget.org, and globalcdn.nuget.org

[Copilot]

Pull request overview

Routes test/CI package resolution away from public NuGet endpoints and through the dnceng dotnet-public Azure Artifacts feed by relying on the repository NuGet.config and updating direct .nupkg downloads.

Changes:

• Removed ExtraNuGetConfigSources overrides pointing at api.nuget.org from generated test projects.
• Updated FilterAssembliesTests direct package downloads to use the dotnet-public V3 flat2 package base address.
• Updated the API docs pipeline to install the boots tool without explicitly adding nuget.org as a source.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
tests/MSBuildDeviceIntegration/Tests/InstallAndRunTests.cs	Removes nuget.org override for a generated device-integration test project.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Tasks/FilterAssembliesTests.cs	Switches direct .nupkg downloads from nuget.org to dotnet-public flat-container URLs.
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest.cs	Removes nuget.org override so restores use the repo NuGet.config feeds only.
build-tools/automation/azure-pipelines-apidocs.yaml	Removes --add-source nuget.org from dotnet tool update so it uses repo feeds after NuGetAuthenticate.

Comments suppressed due to low confidence (1)

tests/MSBuildDeviceIntegration/Tests/InstallAndRunTests.cs:2532

• This test no longer adds nuget.org as an extra source, so package restore relies entirely on the repo NuGet.config feeds. Since this project references Xamarin.Google.Android.InstallReferrer 1.1.2.6 (and the PR description notes it is not mirrored to dotnet-public), the build will fail in network-isolated CI until that package is mirrored (or the test switches to a mirrored package/version).

			var proj = new XamarinAndroidApplicationProject (packageName: PackageUtils.MakePackageName (runtime)) {
				IsRelease = isRelease,
				OtherBuildItems = {
					new AndroidItem.TransformFile ("Transforms\\Metadata.xml") {
						TextContent = () => $@"<metadata><attr path=""/api/package[@name='{gradleModule.PackageName}']"" name=""managedName"">Facebook</attr></metadata>",
					},