Improve Maven POM resolution errors

[jonathanpeppers]

AndroidMavenLibrary resolution failures currently report the originally requested artifact even when a transitive parent or imported POM is missing. This makes repository mismatches difficult to diagnose.

Track the artifact at the failing LoggingPomResolver call and include its exact POM URL in XA4237. The existing localized XA4237 format remains compatible, while the new resolution detail uses a separate localizable resource. Documentation and deterministic imported-BOM coverage are included.

Fixes #9706

• Useful description of why the change is necessary.
• Links to issues fixed.
• Unit tests (MavenDownloadTests: 18 passed).

[Copilot]

Pull request overview

This PR improves diagnostics for <AndroidMavenLibrary> POM resolution by reporting the actual artifact that failed during transitive/parent/imported POM resolution, and by including the exact POM URL in XA4237 to make repository mismatches actionable.

Changes:

• Track the failing artifact inside LoggingPomResolver and emit XA4237 with a new localized “details” payload including unresolved artifact + POM URL.
• Add deterministic unit coverage for imported-BOM failure reporting, and update existing XA4237 message assertions.
• Update XA4237 documentation to reflect the expanded error detail.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Tasks/MavenDownloadTests.cs	Updates XA4237 expectations and adds deterministic test coverage for transitive imported-POM failures (including URL capture).
src/Xamarin.Android.Build.Tasks/Tasks/MavenDownload.cs	Adds unresolved artifact/URL tracking in the POM resolver and enriches XA4237 with localized resolution details.
src/Xamarin.Android.Build.Tasks/Properties/Resources.resx	Adds a new localizable XA4237 details resource used to format resolution failures.
src/Xamarin.Android.Build.Tasks/Properties/Resources.Designer.cs	Updates the generated resource accessor for the new XA4237 details string.
Documentation/docs-mobile/messages/xa4237.md	Updates XA4237 documentation example and guidance to match the new diagnostics.
Documentation/docs-mobile/messages/index.md	Updates the XA4237 index entry to reflect the expanded message shape.

Files not reviewed (1)

• src/Xamarin.Android.Build.Tasks/Properties/Resources.Designer.cs: Generated file

Suppressed comments (1)

src/Xamarin.Android.Build.Tasks/Tasks/MavenDownload.cs:167

• GetRepository uses the raw Repository metadata string for GetKnownRepository, Uri.TryCreate, and cache hashing without trimming whitespace, so values like " https://repo.example.com/maven2/" (or ones with trailing spaces) will fail URL detection and produce XA4239 even though the URL is otherwise valid. It also hashes the unnormalized URL, so a trailing slash creates a separate cache directory despite MavenRepository normalizing it.

		var type = item.GetMetadataOrDefault ("Repository", "Central");
		repositoryUrl = type.TrimEnd ('/');

		var repo = GetKnownRepository (type);
		if (repo == MavenRepository.Central)
			repositoryUrl = "https://repo1.maven.org/maven2";
		else if (repo == MavenRepository.Google)
			repositoryUrl = "https://dl.google.com/android/maven2";

		if (repo is null && Uri.TryCreate (type, UriKind.Absolute, out var uri) &&
			(uri.Scheme == Uri.UriSchemeHttp || uri.Scheme == Uri.UriSchemeHttps)) {
			if (uri.Scheme == Uri.UriSchemeHttp &&