Closes #45046

The root of the issue is anchor resolution. When we apply adjacent
edits, they get merged into a single edit. In the scenario described in
the issue, this is what happens:

1. We create an anchor at the end of each selection (bias::right) on the
snapshot before the edits.
2. We collect the edits and apply them to the buffer.
3. Since the edits are adjacent (>=), the buffer merges them into a
single edit.
4. As a result, we apply one edit to the text buffer, creating a single
visible fragment with length = 3.
5. The buffer ends up with fragments like: [F(len = 3, visible = true),
F(len = 1, visible = false), ...]
6. After the edits, we resolve the previously created anchors to produce
zero-width selections (cursors).
7. All anchors resolve into deleted fragments, so their resolved offset
equals the cumulative visible offset, which is 3.
8. We now have 3 cursors with identical coordinates (0;3).
9. These cursors get merged into a single cursor.

I tried several approaches, but they either felt wrong or didn’t work.
In particular, I tried adjusting anchor resolution using the delta
stored in handle_input, but this doesn’t help because selections are
merged immediately after anchor resolution.

The only workable solution I found is to avoid anchors entirely for the
adjacent-edit case. Instead, we can compute the final cursor positions
directly from the edits and create the selections based on that
information.

Release Notes:

- Fixed an issue where adjacent selection insert would merge cursors

---------

Signed-off-by: Marco Mihai Condrache <52580954+marcocondrache@users.noreply.github.com>
Co-authored-by: Lukas Wirth <me@lukaswirth.dev>