addBrokerToFirm
Adds a broker to an existing active brokerage firm in the caller's partner scope.
The firm must have at least one active organization assignment owned by the caller's partner.
A missing, inactive, unassigned, or other-partner firm returns the same generic NOT_FOUND denial.
addBrokerToFirm(
input: AddBrokerToFirmInput!
): AddBrokerToFirmResponse