Editing a subscription with a free gift item now saves correctly

When a subscription contract included a line item with a 100% discount (such as a free gift), pressing Save on the product details modal appeared to succeed but sent no request to the server. The modal closed, the contract showed no change, and no error message appeared. The form's validation rule rejected 100% as an invalid discount value before the save handler ever ran.

This is fixed. The product details modal now accepts 100% as a valid discount on a subscription line, and saves the contract as expected. Merchants who use free gift line items in their subscriptions can now edit those contracts without any workaround.