upsert method
Implementation
AccumulatedFileStream upsert({required String itemId, String? turnId, String? senderName}) {
final normalizedItemId = itemId.trim();
final current = _items[normalizedItemId];
final next = AccumulatedFileStream(
itemId: normalizedItemId,
turnId: _coalesceText(turnId, current?.turnId),
status: 'in_progress',
urls: List<String>.unmodifiable(current?.urls ?? const <String>[]),
senderName: _coalesceText(senderName, current?.senderName),
);
_items[normalizedItemId] = next;
return next;
}