Add integration tests for third-party toolkit compatibility

[Copilot]

MAUI changes can break compatibility with popular ecosystem packages without being caught by existing tests. This adds integration tests that validate builds with commonly-used third-party toolkits.

Changes

New file: src/TestUtils/src/Microsoft.Maui.IntegrationTests/ToolkitTests.cs

Three test scenarios (6 test cases: Debug + Release each):

• BuildWithCommunityToolkit - Validates CommunityToolkit.Maui integration
• BuildWithSyncfusionToolkit - Validates Syncfusion.Maui.Toolkit integration
• BuildWithMultipleToolkits - Validates both toolkits simultaneously (catches conflicts)

Pattern

Each test:

1. Creates MAUI app from template
2. Injects package references into .csproj
3. Updates MauiProgram.cs with toolkit initialization
4. Builds with TreatWarningsAsErrors=true

Package versions sourced from eng/Versions.props to stay current with tracked versions.

Example

[Theory]
[InlineData("maui", DotNetCurrent, "Debug")]
[InlineData("maui", DotNetCurrent, "Release")]
public void BuildWithCommunityToolkit(string id, string framework, string config)
{
    // Create template project
    DotnetInternal.New(id, projectDir, framework);
    
    // Add toolkit package
    var version = GetPackageVersion("CommunityToolkitMauiPackageVersion");
    FileUtilities.ReplaceInFile(projectFile, "</Project>",
        $"<ItemGroup><PackageReference Include=\"CommunityToolkit.Maui\" Version=\"{version}\" /></ItemGroup></Project>");
    
    // Update MauiProgram.cs
    FileUtilities.ReplaceInFile(mauiProgramFile, 
        ".UseMauiApp<App>()", 
        ".UseMauiApp<App>().UseMauiCommunityToolkit()");
    
    // Build and verify
    DotnetInternal.Build(projectFile, config, properties: BuildProps, msbuildWarningsAsErrors: true);
}

Part of "Build" test category, runs in standard integration test pipeline.

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[dartasen]

I don't think building a project with the package will help avoid the breaking change with InternalVisibleTo. Since the toolkit version will likely target an older version of MAUI than the CI one, it will always build but will cause runtime errors only.

Would creating a sample with several toolkit controls help in some way ?
Just to ensure that the app can build and at least display the first page with various controls without crashing?