supermaven: Fix crash when editing non-ASCII text (#19153)

Closes #19051
Closes #19182


#### How to reproduce this crash:
1. Open any file and input some ASCII characters.
2. Replace these characters with `你好`.
3. Press `backspace`.
4. Crash.



https://github.com/user-attachments/assets/ea5c5340-29a5-42c8-98c5-6e60770445a4



The issue lies with the `prefix_offset` introduced in #18858. After the
buffer is modified, this value is not always valid and may fall within a
`char boundary`, which results in a crash.



Release Notes:

- Fixed Supermaven crashing on deleting non-ASCII text