Add net472 compression reference for packaging tests in legacy Windows build

[Copilot]

The Build and Test (debug, net35, windows-latest) Actions job was failing in the legacy project-load matrix because DocumentFormat.OpenXml.Packaging.Tests compiled as net472 there and no longer resolved System.IO.Compression types used by ZipArchive.

• CI failure root cause

  • The net35/Windows build path maps test projects to net472.
  • OpenXmlPackageTests uses ZipArchive APIs, but the test project did not explicitly reference System.IO.Compression for net472.

• Project fix

  • Add a framework-specific assembly reference in DocumentFormat.OpenXml.Packaging.Tests.csproj.
  • Scope the change to net472 only so other target frameworks remain untouched.

• Resulting behavior

  • Restores compilation for the legacy Windows build matrix without changing test code or product code.

<ItemGroup Condition=" '$(TargetFramework)' == 'net472' ">
  <Reference Include="System.IO.Compression" />
</ItemGroup>