Add host tests for resolving serviceable assets

[elinor-fung]

I realized we didn't have tests around serviceable assets when doing the .deps.json relative local path work. My initial plan for that would have broken these scenarios - hopefully having tests like these would have made that clearer earlier on.

cc @dotnet/appmodel @AaronRobinsonMSFT

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[Copilot]

Pull Request Overview

This PR adds comprehensive host tests for serviceable assets in the .NET Core runtime. The tests verify that the host correctly resolves serviceable assets from the servicing directory while non-serviceable assets are resolved from the application directory.

• Extends the test framework to support configuring serviceable properties on libraries
• Adds three test cases covering runtime assemblies, native libraries, and resource assemblies
• Tests are platform-specific (skipped on Windows due to servicing location requirements)

Reviewed Changes

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

File	Description
src/installer/tests/TestUtils/NetCoreAppBuilder.cs	Adds serviceable property support to RuntimeLibraryBuilder with a new WithServiceable() method
src/installer/tests/HostActivation.Tests/DependencyResolution/ServiceableAssets.cs	New test file containing comprehensive serviceable asset resolution tests

[Copilot]

The ServicedPath property is mutable but lacks validation. Consider making it private set or adding validation to ensure it's only set once by SetUpServicingDirectory.

Suggested change

	public string ServicedPath { get; set; }
	public string ServicedPath { get; private set; }
	public void SetServicedPath(string path)
	{
	if (ServicedPath != null)
	{
	throw new InvalidOperationException("ServicedPath can only be set once.");
	}
	ServicedPath = path;
	}

[Copilot]

Mutating the ServicedPath property of the input parameter creates a side effect that may not be obvious to callers. Consider returning a modified copy or using a different design pattern.

Suggested change

	private string SetUpServicingDirectory(string directory, params LibraryAsset[] libraryAssets)
	{
	string servicingDir = Path.Combine(directory, "servicing");
	foreach (var asset in libraryAssets)
	{
	string servicedAsset = Path.Combine(servicingDir, "pkgs", asset.Name, asset.Version, asset.Path);
	Directory.CreateDirectory(Path.GetDirectoryName(servicedAsset));
	File.WriteAllText(servicedAsset, string.Empty);
	asset.ServicedPath = servicedAsset;
	}
	return servicingDir;
	private (string servicingDir, LibraryAsset servicedServiceableLib, LibraryAsset servicedNonServiceableLib)
	SetUpServicingDirectory(string directory, LibraryAsset serviceableLib, LibraryAsset nonServiceableLib)
	{
	string servicingDir = Path.Combine(directory, "servicing");
	string servicedServiceableAsset = Path.Combine(servicingDir, "pkgs", serviceableLib.Name, serviceableLib.Version, serviceableLib.Path);
	Directory.CreateDirectory(Path.GetDirectoryName(servicedServiceableAsset));
	File.WriteAllText(servicedServiceableAsset, string.Empty);
	string servicedNonServiceableAsset = Path.Combine(servicingDir, "pkgs", nonServiceableLib.Name, nonServiceableLib.Version, nonServiceableLib.Path);
	Directory.CreateDirectory(Path.GetDirectoryName(servicedNonServiceableAsset));
	File.WriteAllText(servicedNonServiceableAsset, string.Empty);
	var servicedServiceableLib = new LibraryAsset(serviceableLib.Name, serviceableLib.Path)
	{
	Version = serviceableLib.Version,
	ServicedPath = servicedServiceableAsset
	};
	var servicedNonServiceableLib = new LibraryAsset(nonServiceableLib.Name, nonServiceableLib.Path)
	{
	Version = nonServiceableLib.Version,
	ServicedPath = servicedNonServiceableAsset
	};
	return (servicingDir, servicedServiceableLib, servicedNonServiceableLib);