Closes #48284

## Summary
- Fix `project_panel::RevealInFileManager` when no project panel entry
is selected.
 - Preserve existing selected entry behavior.
- Add fallback to reveal the last visible worktree root when selection
is empty.
 - Add regression test cov.

## Root Cause
`RevealInFileManager` previously depended on `selected_sub_entry()`.
When selection is cleared (e.g. click project panel background), Command
Palette dispatch had no target and no-op'd.


## Verification
 - `cargo fmt --all -- --check`
 - `./script/check-keymaps`
 - `./script/clippy -p project_panel`
 - `cargo test -p project_panel -- --nocapture`

## Manual Testing
 - Reproduced issue steps from #48284.
- Confirmed Command Palette `Project panel: Reveal in file manager` now
opens project root when selection is empty.
  - Confirmed selected file reveal behavior remains unchanged.
  - Confirmed context menu reveal behavior remains unchanged.

Release Notes:
- Fixed `Project panel: Reveal in file manager` to work even when no
project panel entry is selected.