trello.updateBoardMemberRole changes the role represented by one board membership.
automations/update-trello-board-member-role.automation.ts
board as an ID, short link, or standard board URL, a stable membershipId, and role as "admin", "normal", or "observer". A membership ID is distinct from a member ID. Optional second argument { account } selects a Trello binding.
Returns Signal<{ boardId: string; membershipId: string; role: "admin" | "normal" | "observer" }>.