rope: Assert utf8 boundary of start of `Chunks::new` range (#40253)

We seem to run into panics in related code, so better assert early

Release Notes:

- N/A *or* Added/Fixed/Improved ...
ec0eeaf69dLukas Wirth committed on 10/15/2025, 1:28:51 PM· committed by GitHubparent3763354
1 file changedLine totals unavailable