### Bug report

When the TreeList is set up for Popup editing, the "required" validation triggers immediately when the Popup form is opened.

### Reproduction of the problem

1) Create a Popup editable TreeList.

2) Add the DataAnnotation [Required] attribute to any of the Model properties.

3) Click the built-in "Add new record" command.

4) The editor of the required field is automatically focused and the validation triggers immediately.

The behavior can be observed in the TreeList Popup Editing demo: https://demos.telerik.com/aspnet-core/treelist/editing-popup

### Expected/desired behavior

The validation must trigger on blur, as in the jQuery demo.

### Environment

* **Telerik UI for ASP.NET Core version: 2025.2.520
* **Browser: [all]

Consider the use of TextWriter async methods for the HTML Helpers, for example the WriteInitializationScript methods.  In certain scenarios the use of the synchronous methods causes an exception: System.InvalidOperationException: Synchronous operations are disallowed. Call WriteAsync or set AllowSynchronousIO to true instead.

This can be resolved by explicitly enabling synchronous operations

services.Configure<IISServerOptions>(options =>
    {
        options.AllowSynchronousIO = true;
    });

though synchronous operations have been disabled by default at framework level as of .NET 3.0.