Decomposes a processor-shaped (Stripe- or Fake-produced) Connected
Account payload into a flat attrs map ready for
Accrue.Connect.Account.changeset/2 (D5-02).
Handles both the atom-keyed shape produced by Accrue.Processor.Fake
and the string-keyed shape produced by Accrue.Processor.Stripe (after
Map.from_struct/1), via the same dual-key get/2 helper used by
Accrue.Billing.SubscriptionProjection.get/2.