Add ModelContextProtocol.Core to slnx

Author: halter73

ModelContextProtocol.slnx

@@ -29,6 +29,7 @@
   <Folder Name="/src/">
     <File Path="src/Directory.Build.props" />
     <Project Path="src/ModelContextProtocol.AspNetCore/ModelContextProtocol.AspNetCore.csproj" />
+    <Project Path="src/ModelContextProtocol.Core/ModelContextProtocol.Core.csproj" />
     <Project Path="src/ModelContextProtocol/ModelContextProtocol.csproj" />
   </Folder>
   <Folder Name="/tests/">