Load Directory Members: records with an id update that member, records without create.

Updates only touch fields the record carries: omitting extensions, contact_tags, or comments leaves the stored values unchanged, while an explicit empty value clears them. The whole payload is applied in one transaction. A record whose id matches no existing member in the caller's organization, or an id appearing twice in the payload, fails the upload with a 400 naming the offending record(s); nothing is written.