Swapping a subscription's product no longer fails when no plan matches the new frequency

Swapping a subscription line's product, from the customer portal or from a cancellation-flow swap offer, could fail outright when the contract's billing frequency didn't have a matching selling plan to swap into. The customer would see an error instead of a completed swap.

The swap now completes in this case. The swapped-in line keeps the discount it already had, and the swap picker shows the actual price the swap will save at, rather than a price that didn't match what got written. This only affects contracts on a billing frequency with no selling plan at that frequency; a swap on a contract with a matching plan behaves as before.