OptionalonCalled once per streamed text delta, as tokens arrive.
The incremental text of this chunk.
The full reply accumulated so far, including this chunk.
OptionalonCalled when the worker announces the reply's sources ahead of the text,
so citation chips can render while the reply streams. The done event
carries the final list.
OptionalonCalled when the assistant finishes a tool call mid-stream, so the UI can show a "used tool" affordance before the reply completes.
OptionalsignalCancels the stream when aborted. Cancellation is not an error: the
promise resolves with the partial reply and aborted: true.
Options for ChatApiClient.streamMessage.