OptionalcanShow the user a button to stop the generation (Telegram can_stop).
OptionalkeepKeep the draft visible when the user presses stop (Telegram keep_on_stop).
OptionalonCalled (at most once) when the user stops this draft. Providers without a stop control never call it.
OptionalplatformTransport-native passthrough (e.g. Telegram message_thread_id).
Options of one IProvider.sendDraft call.