streamPaginated abstract method
Paginated stream — emits the latest limit messages, with the ability
to load older pages on demand. App's delegate implementation (Firestore,
Supabase, etc.) is responsible for the actual pagination logic.
Returns a stream of the current "window" of loaded messages. Calling
loadMore expands the window backward (older messages prepended).
Implementation
PaginatedMessageStream streamPaginated(String roomId, {int pageSize = 50});