Disable Validate.csproj build under Mono FullAOT to fix CS0246

[Copilot]

AllSubsets_Mono_MiniFullAot_RuntimeTests build leg has been broken since ~2026-05-01 with CS0246 on using InvalidCSharp; in src/tests/Loader/classloader/generics/ByRefLike/Validate.{cs,csproj}.

Root cause: InvalidCSharp.ilproj is marked MonoAotIncompatible=true, so src/tests/Directory.Build.targets sets DisableProjectBuild=true for it under minifullaot/llvmfullaot. The dependent Validate.csproj was not similarly gated, so the C# compile ran with the IL reference missing.

• src/tests/Loader/classloader/generics/ByRefLike/Validate.csproj: add <MonoAotIncompatible>true</MonoAotIncompatible> (mirrors the existing sibling ValidateNegative.csproj, same Re-enable arm64 and x64 Mono fullAOT llvm and mini jobs #90427 reference).

<PropertyGroup>
  <MonoAotIncompatible>true</MonoAotIncompatible>  <!-- https://github.com/dotnet/runtime/issues/90427  -->
</PropertyGroup>

No loss of FullAOT coverage: the [Fact] methods in Validate.cs are already gated with [ActiveIssue("...90427", ..., IsMonoMINIFULLAOT)] and IsMonoFULLAOT, so they were already skipped at runtime on those variants. This change aligns build-time gating with the existing runtime gating.