… buffer fold (zed-industries#37953)

Closes zed-industries#36376 

Problem:
Multi-cursor edits/selections in multi-buffers view were jumping to
incorrect locations after toggling buffer folds. When users created
multiple selections across different buffers in a multi-buffer view
(like project search results) and then folded one of the buffers,
subsequent text insertion would either:

1. Insert text at wrong locations (like at the top of the first unfolded
buffer)
2. Replace the entire content in some buffers instead of inserting at
the intended cursor positions
3. Create orphaned selections that caused corruption in the editing
experience

The issue seems to happen because when a buffer gets folded in a
multi-buffer view, the existing selections associated with that buffer
become invalid anchor points.

Solution:
1. Selection Cleanup on Buffer Folding
- Added `remove_selections_from_buffer()` method that filters out all
selections from a buffer when it gets folded
- This prevents invalid selections from corrupting subsequent editing
operations
- Includes edge case handling: if all selections are removed (all
buffers folded), it creates a default selection at the start of the
first buffer to prevent panics

2. Unfolding buffers before editing  
- Added `unfold_buffers_with_selections()` call in `handle_input()`
ensures buffers with active selections are automatically unfolded before
editing
- This helps in fixing an edge case (covered in the tests) where, if you
fold all buffers in a multi-buffer view, and try to insert text in a
selection, it gets unfolded before the edit happens. Without this, the
inserted text would override the entire buffer content.
- If we don't care about this edge case, we could remove this method. I
find it ok to add since we already trigger buffer unfolding after edits
with `Event::ExcerptsEdited`.

Release Notes:

- Fixed multi-cursor edits jumping to incorrect locations after toggling
buffer folds in multi-buffer views (e.g, project search)
- Multi-cursor selections now properly handle buffer folding/unfolding
operations
- Text insertion no longer occurs at the wrong positions when buffers
are folded during multi-cursor editing
- Eliminated content replacement bugs where entire buffer contents were
incorrectly overwritten
- Added safe fallback behavior when all buffers in a multi-buffer view
are folded

---------

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>

…of the toggled one (#44394)

Closes #43870

Regressed in #37953

Release Notes:

- Fixed issue where toggling buffer fold focuses first buffer in
multi-buffer instead of the toggled one.

…of the toggled one (zed-industries#44394)

Closes zed-industries#43870

Regressed in zed-industries#37953

Release Notes:

- Fixed issue where toggling buffer fold focuses first buffer in
multi-buffer instead of the toggled one.

…of the toggled one (zed-industries#44394)

Closes zed-industries#43870

Regressed in zed-industries#37953

Release Notes:

- Fixed issue where toggling buffer fold focuses first buffer in
multi-buffer instead of the toggled one.