editor: Fix utf8 boundary panic in `process_completion_for_edit` (#39561)

Fixes ZED-1WH

Release Notes:

- Fixed panic when requesting completions after a multibyte character
da82eec4cbLukas Wirth committed on 10/6/2025, 8:39:51 AM· committed by GitHubparent2bfcd60
1 file changedLine totals unavailable