Fix build warnings in Xamarin.ProjectTools

[simonrozsival]

This PR fixes the following build warning:

 Xamarin.ProjectTools net10.0 succeeded with 1 warning(s) (2.6s) → src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/bin/Debug/net10.0/Xamarin.ProjectTools.dll
   /.../src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Android/AndroidSdkResolver.cs(19,21): warning CA1416: This call site is reachable on all platforms. 'Registry.GetValue(string, string?, object?)' is only supported on: 'windows'. (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1416)

[Copilot]

Pull request overview

Removes a platform analyzer warning (CA1416) from Xamarin.ProjectTools by using an OS guard pattern recognized by the .NET analyzers, and re-enables warnings-as-errors enforcement for that project in CI/local strict builds.

Changes:

• Replace TestEnvironment.IsWindows with OperatingSystem.IsWindows() to properly guard the Windows-only Registry.GetValue call and eliminate CA1416.
• Remove the Directory.Build.props exemption that allowed Xamarin.ProjectTools to build with warnings under TreatWarningsAsErrors.

Show a summary per file

File	Description
src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Android/AndroidSdkResolver.cs	Uses OperatingSystem.IsWindows() to satisfy CA1416 platform guard requirements for Registry.GetValue.
Directory.Build.props	Removes the Xamarin.ProjectTools.csproj warning allowance so warnings become errors when TreatWarningsAsErrors is enabled.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 0