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 "Reveal in file explorer" command doesn't work (project_panel::RevealInFileManager) #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.