End the partnership (a client admin who is not staff of that partner)

Revokes every partner grant (Core access at once, then the Clerk memberships), ends the link and returns billing to the client's own balance. pendingClerkRemovals counts Clerk removals that failed; calling end again retries them. 403 PARTNER_STAFF_CANNOT_END for the partner's own staff, 403 IMPERSONATED_SESSION for an impersonated session; 404 NO_PARTNERSHIP when there is nothing to end.