Update the availability checker to add up all the quantities present in the order

Problem/Motivation

Currently, the availability checker is relying on the order item matcher to match any existing quantities in cart.

Instead, our logic should be much simpler, we should loop over the order lines items, and add-up any quantities for order items referring the same purchased entity.

Doing that will ensure we also support the use-case when order items are not combined, since we do not want to allow less than the minimum quantity configured or more than the max quantity, regardless of the combining behavior.