Fix misleading XA4243 error when more than one JavaArtifact is specified

[Copilot]

MavenExtensions.TryParseJavaArtifact reused XA4243 ("Attribute '{0}' is required...") when a user supplied more than one JavaArtifact — a "missing-attribute" message for what is actually a "too many artifacts" problem. A stale TODO flagged the need for a dedicated message.

Changes

• New error code XA4256 in Resources.resx / Resources.Designer.cs: Only one '{0}' is allowed for '{1}' item '{2}'.
• MavenExtensions.cs: log XA4256 instead of XA4243 on artifacts.Count > 1; removed TODO.

if (artifacts.Count > 1) {
	log.LogCodedError ("XA4256", Properties.Resources.XA4256, "JavaArtifact", type, task.ItemSpec);
	artifact = null;
	return false;
}

[Copilot]

Pull request overview

Fixes an incorrect/misleading MSBuild error code emitted when multiple JavaArtifact values are provided for a single item, by introducing a dedicated error code/message and using it in MavenExtensions.TryParseJavaArtifact.

Changes:

• Added new localized resource string/error code XA4256: “Only one '{0}' is allowed for '{1}' item '{2}'.”
• Updated TryParseJavaArtifact to log XA4256 (instead of XA4243) when more than one artifact is specified.
• Updated Resources.Designer.cs to expose the new XA4256 resource.

Show a summary per file

File	Description
src/Xamarin.Android.Build.Tasks/Utilities/MavenExtensions.cs	Emits XA4256 when JavaArtifact metadata specifies more than one artifact.
src/Xamarin.Android.Build.Tasks/Properties/Resources.resx	Adds the XA4256 localized string.
src/Xamarin.Android.Build.Tasks/Properties/Resources.Designer.cs	Adds the generated accessor for XA4256.

Copilot's findings

Files not reviewed (1)

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

• Files reviewed: 2/3 changed files
• Comments generated: 1