Changes only the supplied fields of a contract job (BF-7482).

Send any of the updatable fields; the rest are left as they are. Null means "leave it", so PATCH cannot clear overtimeRuleId or firstWeekEndingDate - use PUT. Keys that are not updatable fields (statusId, endDate, address, ...) are rejected with 400. clientId and contractorId cannot be changed on an existing job. Rules on the resulting job are enforced only for fields that change. addressId and stateId are the exception to "null means leave it": null clears the address (a state cannot be cleared). A new address must be an active address of the job's client and gives the job its state.