diagnostics: Fix losing focus when activating from diagnostics view (#25517)

Closes #25509

Changes:
- If active item is already diagnostics, don't try to focus it again.

Instead of not focusing, should it just not activate instead? Something
like:

            if !workspace
                .active_item(cx)
                .map(|item| item.item_id() == existing.item_id())
                .unwrap_or(false)
            {
workspace.activate_item(&existing, true, true, window, cx);
            }


Release Notes:

- N/A