Fix Korean IME overlay hiding characters to the right of cursor (#20041)

This commit fixes a regression in v1.24, where composing Korean text
in-between existing characters causes text to the right to be hidden.
This is problematic for the Korean IME, because it commonly inserts
the composed syllable between existing characters.

The fix compresses (removes/absorbs) available whitespace to the right
of the composition to make space for any existing text. This means
that a composition now virtually acts in insert mode.

Closes #20040
Refs #19738
Refs #20039

## Validation Steps Performed

1. Korean IME: compose `ㄷ` between `가` and `나` → display shows `가ㄷ나`,
`나` visible.
2. TUI box (vim prompt with padding): compose inside padded text box →
border preserved in place.
3. Composition at end of line (no chars to right): unaffected.
4. English and other IME input: not affected (no active composition
row).

Co-authored-by: jason <drvoss@users.noreply.github.com>
Co-authored-by: Leonard Hecker <lhecker@microsoft.com>