Use conditional SetTargetFramework in existing test project instead of separate project

On non-Windows platforms, the net8.0 test target now forces netstandard2.0 build of ModelContextProtocol.Core via conditional SetTargetFramework. This provides cross-platform netstandard2.0 test coverage without requiring a separate test project.

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

Author: Copilot

ModelContextProtocol.slnx

@@ -73,7 +73,6 @@
     <Project Path="tests/ModelContextProtocol.AspNetCore.Tests/ModelContextProtocol.AspNetCore.Tests.csproj" />
     <Project Path="tests/ModelContextProtocol.ConformanceClient/ModelContextProtocol.ConformanceClient.csproj" />
     <Project Path="tests/ModelContextProtocol.ConformanceServer/ModelContextProtocol.ConformanceServer.csproj" />
-    <Project Path="tests/ModelContextProtocol.Netstandard2_0.Tests/ModelContextProtocol.Netstandard2_0.Tests.csproj" />
     <Project Path="tests/ModelContextProtocol.TestOAuthServer/ModelContextProtocol.TestOAuthServer.csproj" />
     <Project Path="tests/ModelContextProtocol.Tests/ModelContextProtocol.Tests.csproj" />
     <Project Path="tests/ModelContextProtocol.TestServer/ModelContextProtocol.TestServer.csproj" />

tests/ModelContextProtocol.Netstandard2_0.Tests/ModelContextProtocol.Netstandard2_0.Tests.csproj

@@ -78,7 +78,10 @@
   </ItemGroup>
 
   <ItemGroup>
-    <ProjectReference Include="..\..\src\ModelContextProtocol.Core\ModelContextProtocol.Core.csproj" />
+    <ProjectReference Include="..\..\src\ModelContextProtocol.Core\ModelContextProtocol.Core.csproj">
+      <!-- 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" />
   </ItemGroup>