hostSlotBuilder property
Builder the host registers to render host_slot rich-content components
with its own native widget. The bot reserves a spot (slot_id) and the
SDK calls this with (slotId, payload); return the widget to inline in
the bubble (or null for unrecognised slot ids → renders nothing).
Use it for app-owned inline UI the chat server can't draw — e.g. an
in-chat PDF preview of a generated document, or a design picker with real
template thumbnails. Must render purely from payload: chat history
reloads re-parse the message JSON, so nothing may be keyed on local state.
Implementation
final HostSlotBuilder? hostSlotBuilder;