diagnostics: Fix losing focus when activating from diagnostics view

[5brian]

Closes #25509

Changes:

• Check if active item is already diagnostics, and set focus_item to false if so.

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

[Anthony-Eid]

Thank you, I'm going to look into your activation question and see if I can get this merged in later today