Cart quantity limits are now determined by market region

Cart quantity limits now depend on inventory available in the buyer's market region, not total stock. Previously, customers could add up to the total product inventory from all locations to their cart. Now, if a customer attempts to add more items than are available for fulfillment in their market, the quantity will automatically be reduced to the available amount and the API will return a message to explain the adjustment.

This change applies to shops with fulfillable inventory set to sell only within configured shipping zones and impacts cart operations in the GraphQL Storefront API.