FileDialog loses multiple file selection when refreshed

### Tested versions

- Reproducible in: 4.7.1.stable and 4.3.stable

### System information

Godot v4.7.1.stable (a13da4feb) - Arch Linux #1 SMP PREEMPT_DYNAMIC Sat, 18 Jul 2026 17:30:57 +0000 on X11 - X11 display driver, Multi-window, 2 monitors - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 4060 Laptop GPU - 13th Gen Intel(R) Core(TM) i7-13650HX (20 threads) - 15.31 GiB memory - PulseAudio (44100 Hz, Stereo/mono)

### Issue description

When FileDialog has multiple files selected, and refresh button is pressed, the refresh state is lost and only one selection stays. Expected behaviour is that selection would remain the same, as it is the case with single file selections.

### Steps to reproduce

1. Open MRP
2. Press "Open" button which will open FileDialog
3. Select 2 or more files
4. Press refresh button
5. Notice that selection has reduced to 1 now

### Minimal reproduction project (MRP)

[godot-mvp-bug-260731-file-dialog-refresh.zip](https://github.com/user-attachments/files/30576566/godot-mvp-bug-260731-file-dialog-refresh.zip)

Start with the attached minimal reproduction project and reproduce the selection loss in FileDialog by selecting multiple files and pressing refresh. Trace the FileDialog refresh handling in the Godot source, then verify that all selected files remain selected after refresh while preserving the existing single-selection behavior.

Written by the indexing model from the issue text.