[desktop_webview_window]: addOnUrlRequestCallback not triggered on SPA (e.g. React) client-side route changes

Hi, thank you for this helpful plugin!

I'm using desktop_webview_window in my Flutter Linux app to embed a webview. The addOnUrlRequestCallback works fine for traditional full-page navigations, but it's not triggered when navigating within a Single Page Application (SPA) such as a React app that uses client-side routing (e.g. React Router or Next.js).

Steps to Reproduce:

1. Load a React-based SPA (e.g. a local or deployed React app) in the webview.
2. Use the app's navigation (e.g. Use navigate or clicking on a element to route to another page).
3. Observe that addOnUrlRequestCallback is not called on these URL changes.

Expected Behavior:

1. The callback should also be triggered when the URL changes due to client-side routing in SPAs, not just for full-page navigations.
2. Alternatively, we could have a new dedicated callback specifically for detecting URL changes in SPA apps.

Actual Behavior:
addOnUrlRequestCallback is only triggered for full page loads (e.g. window.location.href = '...') and not for history API-based routing (history.pushState, replaceState), which is commonly used in SPAs.

Environment:
OS: Linux
Flutter version: 3.29.2
Plugin version: ^0.2.3

No source file or test is named. Reproduce the issue in a Flutter Linux app with a React SPA, then trace the desktop_webview_window handling behind addOnUrlRequestCallback and history.pushState/replaceState; done means SPA route changes are detected through the agreed callback behavior and covered by a regression test.