VariantAttribute - OrderMesh DEV

The canonical name/value attribute shape shared across the variant family (variant attrs/opts/extras
and per-decoration-area attributes on both global and merchant variants). PROD-935: unifies the
previously separate, id-bearing attribute and extra shapes into one id-less shape. [BsonIgnoreExtraElements]
makes the gradual migration explicit at the type level — stored docs that still contain id/value_id
deserialize fine without relying on the global convention.