Get Prefund balance

Returns the current state of the Partner's Prefund Account. This endpoint is partner-scoped, not user-scoped; UR maintains one Prefund Account per partner, and the response is a flat, single-currency object for the settlement currency (defaults to USD). Poll at operational dashboard cadence, e.g. no more frequently than once per minute unless UR advises otherwise and do not call it from inside the Card Authorization callback; UR already performs Prefund availability checks before approving.