Creates a new Microsoft private offer from a full offer payload, or
imports an existing live Microsoft private offer when the body contains
microsoftOfferId and buyer_company_name.

For full create payloads, if status is submit_to_cloud, the handler
validates and transforms the request for Microsoft submission before
saving the offer. For import payloads, the vendor context is derived
from the authenticated principal; sandbox vendors use their production
parent vendor's Microsoft credentials.

Required RBAC action: offers:CreateDraftOffer. Submitting to Microsoft
also requires offers:CreateOfferOnMarketplace.