Template context handed to the busy (*cngxAccordionItemBusy) and error
(*cngxAccordionItemError) slots so an override can read the item's live
state without re-injecting it - parity with
CngxAccordionItemIconContext.
$implicit is the current AsyncStatus (loading / refreshing in
the busy slot, error in the error slot). message carries the resolved
error string in the error state and is empty in the busy state, so an error
override can render the announced text (and a retry) without re-deriving it.