droppingLowestPriorityContextItem

Returns a copy without its least-important context item, plus the item dropped.

Re-read prompt after each drop until the request fits:

var request = request
while (tooLong(request.prompt())) {
    request = request.droppingLowestPriorityContextItem()?.first ?: break
}

The trimmed request and the dropped item, or null when the context is already empty.