complete method
Implementation
AccumulatedFileStream? complete(String itemId, {String status = 'completed'}) {
final current = _items[itemId.trim()];
if (current == null) {
return null;
}
final next = AccumulatedFileStream(
itemId: current.itemId,
turnId: current.turnId,
status: status,
urls: current.urls,
senderName: current.senderName,
);
_items[current.itemId] = next;
return next;
}