Clicking on a sticky header causes
`selections.select_ranges([anchor..anchor])` to be called, but this does
not clear the editor's `selections.select_mode()`, resulting in possible
incorrect selections if this is followed up by a shift-click. This PR
fixes that with
```diff
- selections.select_ranges([anchor..anchor]);
+ selections.clear_disjoint();
+ selections.set_pending_anchor_range(anchor..anchor, SelectMode::Character);
```
which is essentially what `editor.select(SelectPhase::Begin { ... },
...)` (i.e. a regular single click in the editor) does as well.

Before:


https://github.com/user-attachments/assets/bcf2647e-a22a-4866-8975-d29e135df148

After:


https://github.com/user-attachments/assets/fb82db51-fef1-4b7c-9954-6e076ae0b176

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Fixed bug that caused clicking on a sticky header to not always
properly clear the previous selection.