Revert "Remove platform-specific DefaultTestTargetFrameworks"

This reverts the changes from commit db11c07. The approach of removing net472 on Windows was incorrect. We need to maintain both:
1. net472 tests on Windows (for netstandard2.0 testing on .NET Framework)
2. Conditional SetTargetFramework on non-Windows (for netstandard2.0 testing on modern .NET)

Restoring the configuration where:
- Windows tests: net10.0, net9.0, net8.0, net472
- Non-Windows tests: net10.0, net9.0, net8.0 (with net8.0 using netstandard2.0 build)

Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

Author: Copilot

Directory.Build.props

@@ -47,7 +47,8 @@
   </PropertyGroup>
 
   <PropertyGroup>
-    <!-- Shared target frameworks for test projects and test support projects. -->
+    <!-- Shared target frameworks for test projects and test support projects. net472 is only included on Windows. -->
     <DefaultTestTargetFrameworks>net10.0;net9.0;net8.0</DefaultTestTargetFrameworks>
+    <DefaultTestTargetFrameworks Condition="$([MSBuild]::IsOSPlatform('Windows'))">$(DefaultTestTargetFrameworks);net472</DefaultTestTargetFrameworks>
   </PropertyGroup>
 </Project>

tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj

@@ -79,8 +79,8 @@
 
   <ItemGroup>
     <ProjectReference Include="..\..\src\ModelContextProtocol.Core\ModelContextProtocol.Core.csproj">
-      <!-- Force net8.0 tests to use netstandard2.0 build for netstandard2.0 test coverage on all platforms -->
-      <SetTargetFramework Condition="'$(TargetFramework)' == 'net8.0'">TargetFramework=netstandard2.0</SetTargetFramework>
+      <!-- On non-Windows platforms, force net8.0 tests to use netstandard2.0 build for cross-platform netstandard2.0 coverage -->
+      <SetTargetFramework Condition="'$(TargetFramework)' == 'net8.0' AND !$([MSBuild]::IsOSPlatform('Windows'))">TargetFramework=netstandard2.0</SetTargetFramework>
     </ProjectReference>
     <ProjectReference Include="..\ModelContextProtocol.TestServer\ModelContextProtocol.TestServer.csproj" />
     <ProjectReference Include="..\..\samples\TestServerWithHosting\TestServerWithHosting.csproj" />