Vouch request: fix #13195 (macOS input-source switch inserts CJK preedit as text)

[Nagato-Yuzuru]

What do you want to change?

I'd like to fix #13195. My plan see #13193 (comment)

Why do you want to make this change?

I hit this daily as a CJK + nvim user. I recently switched from Wezterm to Ghostty.

Other terminals handle it correctly. Since I can already reproduce it and understand the code path, I'd like to fix it myself.

I acknowledge that:

• I have read the Contributing Guide and understand the contribution process.
• I have read and agree to follow the AI Usage Policy.
• I wrote this vouch request myself, in my own voice, without AI generating it.

[bo2themax]

!vouch