Surrogate pair characters (emoji, rare CJK, etc.) break a, r, s, ~, ga commands

[k1832]

Describe the bug

When the cursor is on a character encoded as a UTF-16 surrogate pair — emojis (😄), rare CJK characters (𩸽), musical symbols (𝄞), etc. — several character-level commands produce incorrect results.

Command	Broken behavior
a (Append)	Cursor lands before the character instead of after
r (Replace)	Only replaces half the character, corrupting the text
s (Change char)	Only deletes half the character before entering Insert mode
~ (Toggle case)	Corrupts the character into a lone surrogate
ga (Unicode info)	Shows the half-surrogate value instead of the full codepoint

To Reproduce

1. Open a file containing 😄text
2. Place cursor on 😄
3. Press a (append), type !, press Esc
4. See !😄text — ! inserted before the emoji instead of after

Expected behavior

😄!text — ! should be inserted after the emoji.

Environment (please complete the following information):

• Extension (VsCodeVim) version: 1.32.4
• VSCode version: 1.109.0
• OS: Ubuntu 24.04

Additional context

position.getRight() increments by 1 UTF-16 code unit, but these characters are 2 code units (a surrogate pair). Moving by 1 lands between the pair, and VSCode's validatePosition clamps it back to the start.

x/X, l/h, and y already have surrogate boundary correction and work correctly.