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 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.