Fix grouping key in ViewModel registrations

Changed the grouping key from SplatRegistrationType to SplatViewModelRegistrationType in viewModelRegistrations to ensure correct grouping of ViewModel registration types.

Author: ChrisPulman

src/ReactiveUI.SourceGenerators.Roslyn/IViewFor/IViewForGenerator.Execute.cs

@@ -337,7 +337,7 @@ private static string GenerateRegistrationExtensions(in ImmutableArray<IViewForI
 
         var viewModelRegistrations = iviewForInfo
             .Where(static x => !string.IsNullOrWhiteSpace(x.SplatViewModelRegistrationType))
-            .GroupBy(static x => (x.TargetInfo.TargetNamespaceWithNamespace, x.ViewModelTypeName, x.SplatRegistrationType))
+            .GroupBy(static x => (x.TargetInfo.TargetNamespaceWithNamespace, x.ViewModelTypeName, x.SplatViewModelRegistrationType))
             .Select(static g => g.First())
             .ToImmutableArray();