Assign model binder strong type only when target type is matched

Author: daningalla

src/lib/Vertical/Cli/Invocation/HandlerContextBuilder.cs

@@ -61,7 +61,7 @@ internal HandlerContextBuilder(ICommand command, CommandHandler<TModel> handler)
     private Func<TModel>? _activator;
 
     /// <inheritdoc />
-    public override void AddModelBinder(Delegate modelBinder) => _modelBinder = modelBinder as ModelBinder<TModel>;
+    public override void AddModelBinder(Delegate modelBinder) => _modelBinder ??= modelBinder as ModelBinder<TModel>;
 
     /// <inheritdoc />
     public override void AddModelActivator(Delegate constructor) => _activator = constructor as Func<TModel>;