Applying exponent to an already superscripted base produces invalid LaTeX #3099

Steps to reproduce

1. Type 2 in a mathfield and select it.
2. Open the virtual keyboard and click the square button.
3. Select only the base 2, excluding its exponent.
4. Click the square button again.

Actual behavior

Before export, the expression visually resembles {2^2}^2.
However, getValue() returns 2^2^2, while errors is empty.
Reloading this value displays it like 2^{22}.
The exported LaTeX is invalid: TeX/KaTeX reject it with “Double superscript”.

Expected behavior

The operation should preserve a valid expression whose appearance and structure survive export/reload.

Reproduced with MathLive 0.111.0 in all major browsers using a standalone mathfield.