Fix abbreviation expansion inserting a double trailing space

[MerlijnW70]

Summary

Expanding an abbreviation by pressing <space> inserted two trailing
spaces instead of one — one before the cursor and one after. This breaks
history-based completion: e.g. git switch (two spaces) is not a prefix of
git switch main, so the ghost-text completion disappears until the extra
space is deleted manually. Fixes #1108.

Fix

On a <space> expansion the triggering space has already been inserted into
the buffer before try_expand_abbreviation_at_cursor runs. The replacement
selected only the abbreviation word (word_start..word_end), leaving that
space in place, and then inserted the expansion followed by its own suffix
space — yielding two trailing spaces.

The selection now extends through cursor_position_in_buffer instead of the
end of the word, so the already-inserted triggering space is replaced by the
expansion's suffix. The submit (<enter>) path is unaffected: there the cursor
sits at the end of the word, so the selected range is unchanged.

Test

Adds abbreviation_expands_on_space_without_double_space, which exercises the
<space> expansion path (submitted = false) — previously untested; all
existing abbreviation tests only covered the submit path. It asserts that
expanding gc → git commit after the triggering space yields exactly
git commit (one trailing space), and fails against the old behaviour
(git commit ).

Reference

Reported in #1108. The issue pinpointed src/engine.rs abbreviation handling
as the cause.

[fdncred]

lgtm. what do you think @kronberger-droid ?

[kronberger-droid]

Seems fine to me to.
I am just testing it and check how/if it interacts with my #1109.
I will keep you all updated.

[kronberger-droid]

Found some things which are orthogonal to this change. Fine to land.

[fdncred]

Thanks!

[MerlijnW70]

@fdncred your welcome,