Ensure IsExternalInit is type forwarded on NET builds (#619)

It's included as internal on netstandard/net472, and the C# compiler may bake a reference to that into a consumer. If that consumer is then used with a net8.0+ build, the IsExternalInit needs to be there and forwarded to the real one.

This switches our polyfill files to always be included but the actual contents ifdef'd out on TFMs that already have the contents. That then makes it easier to do specialized ifdef'ing in the future, as this does for IsExternalInit.

src/ModelContextProtocol.Core/ModelContextProtocol.Core.csproj

@@ -20,7 +20,6 @@
 
   <!-- Dependencies only needed by netstandard2.0 -->
   <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
-    <Compile Include="..\Common\Polyfills\**\*.cs" />
     <Compile Include="..\Common\CancellableStreamReader\**\*.cs" />
     <PackageReference Include="Microsoft.Bcl.Memory" />
     <PackageReference Include="System.Diagnostics.DiagnosticSource" />