fix: Update src/Mapster.JsonNet/Mapster.JsonNet.csproj

Co-authored-by: DevTKSS <[email protected]>

Author: DocSvartz

src/Mapster.JsonNet/Mapster.JsonNet.csproj

@@ -4,6 +4,7 @@
     <TargetFrameworks>net10.0;net9.0;net8.0;</TargetFrameworks>
     <Description>Json.net conversion supports for Mapster</Description>
     <IsPackable>true</IsPackable>
+    <IsTestProject>true</IsTestProject>
     <PackageTags>Mapster;Json.net</PackageTags>
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>Mapster.JsonNet.snk</AssemblyOriginatorKeyFile>