Add `ExpandAllEntries` action, keybinding, and right-click context menu
entries for expand/collapse all in the project panel.

- Add `ExpandAllEntries` action + handler + `expand_all_entries()`
method
- Add `cmd-right` / `ctrl-right` keybinding for `ExpandAllEntries`,
mirroring
  the existing `cmd-left` for `CollapseAllEntries`
- Add "Expand All" to root-entry and folder right-click context menus
- Add keybinding hint to root-entry "Collapse All" context menu entry
- Per-worktree behavior: context menu actions affect only the selected
worktree,
  subfolder "Expand All" expands from that folder down

# Objective

The project panel had no discoverable way to expand or collapse all
entries. The only ways were keyboard shortcuts (`cmd-left` for collapse
all)
or the right-click context menu. The global `ExpandAllEntries` shortcut
(`cmd-right`) was also missing. This PR adds that shortcut and exposes
both actions in the context menu so users learn the keybindings while
keeping the UI minimal.

## Solution

- Add `ExpandAllEntries` action + handler + `expand_all_entries()`
method
- Add `cmd-right` / `ctrl-right` keybinding for `ExpandAllEntries`
(global),
  mirroring the existing `cmd-left` for `CollapseAllEntries`
- Add "Expand All" to the right-click context menu for both root entries
and
  subfolder entries
- Update root-entry "Collapse All" to show its keybinding hint via
  `.action(Box::new(CollapseAllEntries))`
- **Per-worktree behavior**: each root entry's context menu actions
affect
only that worktree, subfolder "Expand All" expands from that folder down

## Testing

- Added 6 GPUI tests in `project_panel_tests.rs`:
  - `test_expand_all_entries`: single worktree
- `test_expand_all_entries_multiple_worktrees`: global expand across
worktrees
  - `test_expand_all_entries_via_window_dispatch`: action dispatch path
  - `test_per_worktree_expand`: expand only the clicked worktree
- `test_per_worktree_collapse`: collapse only the clicked worktree, keep
root visible
  - `test_expand_all_entries_with_auto_fold`: expand with auto_fold_dirs
- Ran `cargo test -p project_panel`: 106 pass

**Manual testing for reviewers:**

1. Open a project with nested directories and open the Project Panel
2. Right-click a root entry, confirm "Expand All" and "Collapse All"
appear
   with their keybinding hints (`⌘→` / `⌘←`)
3. Right-click a subfolder, confirm "Expand All" and "Collapse All"
appear
4. Click "Expand All" on a subfolder, confirm only that folder's subtree
expands
5. Click "Collapse All", confirm children collapse but the root stays
visible
6. Open a multi-folder workspace. Right-click the second root and click
   "Expand All", confirm only that folder expands
7. Press `cmd-right` (or `ctrl-right`), confirm all worktrees expand
globally

Tested on macOS only. Linux and Windows should behave the same since
there
is no platform-specific code.

## 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 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)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Showcase



https://github.com/user-attachments/assets/adc501d7-8e3a-4687-aef0-dc00426d232d

---

Release Notes:

- Added `Expand All` and `Collapse All` to the project panel right-click
  context menu with keybinding hints.
- Added `cmd-right` / `ctrl-right` keybinding to expand all entries.

---------

Co-authored-by: Danilo Leal <daniloleal09@gmail.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>