## Context

Shift-click sometimes extended a selection from an earlier word boundary
after double-clicking a word and moving the cursor with the arrow keys.
Cursor movement changed the selection but left the word or line
selection mode active, so the next Shift-click reused stale selection
bounds. Selection movement now resets the mode to character mode
whenever it changes the selection, while preserving valid empty line
selections that have not been moved.

Closes #59913.

Behavior before the fix :

[Screencast from 2026-07-15
13-21-51.webm](https://github.com/user-attachments/assets/c126adfe-f9db-4a04-91c2-6db6e8070182)

Behavior after the fix :

[Screencast from 2026-07-15
13-19-38.webm](https://github.com/user-attachments/assets/19c9f5b6-aaf8-4e8b-b4c5-70327411d1b9)


## How to Review

**`crates/editor/src/selections_collection.rs`**:
`MutableSelectionsCollection::move_with` now resets the selection mode
to character mode when movement changes a selection.

**`crates/editor/src/editor_tests.rs`**: Regression tests cover
Shift-click after collapsing a word selection and confirm that a valid
empty line selection retains line-wise extension behavior.

## Self-Review Checklist

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the UI/UX checklist
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Fixed Shift-click sometimes extending selections from a previous word
or line boundary.