Called by a cart-recovery Journey immediately before each outbound step (SMS, Email, Voice,
…) to confirm the shopper still hasn't converted, edited their cart, or otherwise moved past
the abandoned-checkout state the Journey was triggered for.

cart_activity_id is the opaque identifier returned as cart_activity_id in the Journey
trigger's variables payload for this checkout — it is not derived from any other field and
should be passed through as-is.