The Thumbnail() option of the ImageBrowser does not provide UrlHandler() option

[ivaylo-milanov]

Bug report

The Thumbnail() option of the Editor's ImageBrowser configuration does not provide UrlHandler() option, which is available for the Image() option.
Referenced in this commit

Reproduction of the problem

1. Define an Editor and enable the ImageBrowser feature:
   Open this REPL example

Expected/desired behavior

The UrlHandler() function must be available to set the URL that serves the thumbnail using a JavaScript handler.

TicketID:

1682129

Environment

• **Telerik UI for ASP.NET Core version: 2025.1.227
• **Browser: [all]