Virtual keyboard does not work when fullscreen on mobile devices

### Tested versions

4.6.1

### System information

Firefox 148.0.1 or Chrome 145.0.X.X on Android 13 (samsung S20)

### Issue description

Any project exported for the web that is shown fullscreen will fail to bring up the OS virtual keyboard when a lineedit is focused. It works fine in windowed mode.

Tested on chrome and firefox, I don't have an apple device to test safari on.

I guess it is possible that this is could be an OS or browser bug and not godot's responsibilty?

For the moment I have hacked around it by turning fullscreen off/on in the `LineEdit.editing_toggled` signal handler.

See #89221

### Steps to reproduce

1. Open the MRP
2. export it for web
3. serve it via some webserver (docker compose included)
4. browse to it on a mobile device
5. hit the "fullscreen" button
6. try to edit the lineedit text

### Minimal reproduction project (MRP)

[virtual-keyboard-fullscreen.tar.gz](https://github.com/user-attachments/files/25888181/virtual-keyboard-fullscreen.tar.gz)

Start with the attached MRP, export it for the web, serve it using the included docker-compose setup, and reproduce the LineEdit behavior on a mobile device in fullscreen. Compare fullscreen and windowed mode in Chrome and Firefox, then verify that the OS virtual keyboard opens for the LineEdit without relying on the editing_toggled workaround.

Written by the indexing model from the issue text.