The unstaged diff view introduced in the partially staged changes commit
only had a Stage button but no Restore option. This adds:

- A per-hunk Restore button in the inline hunk controls (disabled for
new files)
- A Restore button in the toolbar for selected hunks
- A Restore All button in the toolbar to discard all unstaged changes
- A restore method override on UnstagedDiffDelegate for the Restore
action

Release Notes:

- Added restore buttons to the unstaged diff view for discarding
unstaged changes.

# Objective
New unstaged and staged diffs were added in #46541 . However for
unstaged changes, only option available is to stage a change. It would
also be a common use case to restore the changes. (Like how it is done
in uncommitted changes).

## Solution
- A per-hunk Restore button in the inline hunk controls (disabled for
new files)
- A Restore button in the toolbar for selected hunks
- A Restore All button in the toolbar to discard all unstaged changes
- A restore method override on UnstagedDiffDelegate for the Restore
action

## Testing
Tested in locally and ensured both features work

## Self-Review Checklist:

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

## Showcase
<img width="1141" height="635" alt="Screenshot 2026-07-09 at 9 00 47 AM"
src="https://github.com/user-attachments/assets/b1d3f091-9f1d-49c6-87e6-9efad25e62dd"
/>

## Note
I am still learning rust bit by bit. Please let me know if something is
massively wrong. This is assisted by AI but reviewed by me with best of
my knowledge.

---

Release Notes:

- Added restore buttons to the unstaged diff view for discarding
unstaged changes.

---------

Signed-off-by: Pranav <pranav10121@gmail.com>
Co-authored-by: Christopher Biscardi <chris@christopherbiscardi.com>