appendDelta method
Implementation
AccumulatedTextStream appendDelta({required String itemId, required String delta, String? turnId, String? senderName, String? phase}) {
final current = upsert(itemId: itemId, turnId: turnId, senderName: senderName, phase: phase);
final next = AccumulatedTextStream(
itemId: current.itemId,
turnId: current.turnId,
status: 'in_progress',
text: accumulateTextStreamDelta(current.text, delta),
senderName: current.senderName,
phase: current.phase,
);
_items[current.itemId] = next;
return next;
}