Fixes movement::find_boundary to work on the buffer, not on display
points.

The user-visible impact is that the "until end of word" commands now
correctly go to the end of a soft-wrapped word (instead of to the first
character of the wrapped line).

It also fixes a bug where the callback passed to these methods was
called with the content of inlay hints.

[[PR Description]]

Release Notes:

- fix finding end of word on soft-wrapped lines