Explicitly handle char marshalling in OleVariant::GetNativeMet… (dotnet/coreclr#26218)

Explicitly handle char marshalling in OleVariant::GetNativeMet… (dotnet/coreclr#26218)

* Explicitly handle char marshalling in OleVariant::GetNativeMethodTableForVarType.

* Added test case for ByVal Unicode Char Array.

* Fix contracts and remove unneeded GC transition.

Commit migrated from https://github.com/dotnet/coreclr/commit/72ceb36dbd4777461740e25099c83d9be5b3a7ff