Properly normalize completion labels with multi-byte characters (#25150)

Closes https://github.com/zed-industries/zed/issues/25142

Release Notes:

- Fixed panics on completion with multi-byte characters input
094430e5a2Kirill Bulatov committed on 2/19/2025, 12:11:10 PM· committed by GitHubparentd4414ef
1 file changedLine totals unavailable