Fix test project duplicate compile for GuidHelpers.cs on net472

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj

@@ -34,14 +34,17 @@
     <Compile Include="..\..\src\ModelContextProtocol.Core\Server\InMemoryMcpTaskStore.cs" Link="Server\InMemoryMcpTaskStore.cs" />
     <!-- Include dependencies for the linked InMemoryMcpTaskStore.cs -->
     <Compile Include="..\..\src\Common\Experimentals.cs" Link="Experimentals.cs" />
-    <Compile Include="..\..\src\Common\Polyfills\System\GuidHelpers.cs" Link="Polyfills\GuidHelpers.cs" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFramework)' == 'net472'">
     <Compile Include="..\..\src\Common\Throw.cs" Link="Throw.cs" />
     <Compile Include="..\..\src\Common\Polyfills\**\*.cs" />
   </ItemGroup>
 
+  <ItemGroup Condition="'$(TargetFramework)' != 'net472'">
+    <Compile Include="..\..\src\Common\Polyfills\System\GuidHelpers.cs" Link="Polyfills\GuidHelpers.cs" />
+  </ItemGroup>
+
   <ItemGroup>
     <PackageReference Include="coverlet.collector">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>