LineEdit: cannot extend selection that was set in script

### Tested versions

- Reproducible in: 4.5.1-stable, 4.6-stable

### System information

Godot v4.6.stable unknown - Arch Linux #1 SMP PREEMPT_DYNAMIC Sat, 24 Jan 2026 00:47:39 +0000 on Wayland - X11 display driver, Single-window, 1 monitor - OpenGL 3 (Compatibility) - Mesa Intel(R) Iris(R) Xe Graphics (ADL GT2) - 12th Gen Intel(R) Core(TM) i5-12500H (16 threads) - 15.33 GiB memory

### Issue description

In some cases, you would wish to set the selection in a LineEdit through script and allow the user to extend/shrink it later as they see fit. However, it seems that keyboard selection and script selection do not match up.

### Steps to reproduce

1. Set both the selection and caret position on a focused `LineEdit` with `LineEdit.select()` and `LineEdit.caret_column` respectively, making sure that caret aligns with either selection start or selection end.
2. Try to adjust the selection with `[Shift] + [Left Arrow]` or `[Shift] + [Right Arrow]`.

Expected behaviour: current selection is expanded/shrunk, depending on caret position and action.

Observed behaviour: new selection is created starting from column 0 to `caret_column`.

### Minimal reproduction project (MRP)

Test project: [line-edit-test.zip](https://github.com/user-attachments/files/24991406/line-edit-test.zip)

Open the scene: you would see a LineEdit with text '12345' with '5' selected and caret between '4' and '5'. Press `[Shift] + [Left Arrow]`. Instead of expanding the selection to be '45', it starts from the beginning of the string to be '123'.

Run the linked minimal reproduction project and inspect the LineEdit.select(), caret_column, and Shift+Arrow selection behavior described in the report. The work is complete when a selection set by script can be expanded or shrunk from either aligned caret position without creating a selection from column 0.

Written by the indexing model from the issue text.