Fix autosave on focus change error when switching git diff via command palette

[OmChillure]

Summary of whats fixed:

• Fixed the autosave regression where focus change to the command palette could trigger autosave at the wrong time in Vim/Helix workflows.
• The behavior is now: defer autosave while command palette is active, then decide on dismiss:
• Skip autosave if focus returns to the same item.
• Run autosave if focus moved elsewhere.

Files changed:

• Added deferred autosave queueing when focus leaves an item to command palette in crates/workspace/src/item.rs.
• Added a modal dismissed event so workspace can react after modal close in crates/workspace/src/modal_layer.rs.
• Wired workspace to process deferred autosaves on modal dismiss and added handling logic in crates/workspace/src/workspace.rs.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Closes #53863

Video

Screencast.from.2026-04-14.22-57-12.webm

Release Notes:

• Fixed autosave-on-focus-change behavior around command palette dismissal in Vim/Helix mode by covering deferred autosave semantics (skip when focus returns to same item, save when focus moves elsewhere).

[ConradIrwin]

Thanks for this!

I'm going to try and attack the problem from a more holistic angle (seems like #45166 introduced a number of issues; and #51949 fixes some of them). It seems better to take the approach from this PR of deferring auto-saves and use it more consistently, avoiding the need for most of the logic in #51949.