chore(checkout): CHECKOUT-7445 Update consignment interface

[bc-mackxu]

What/Why?

Add comparisonShippingCost and shippingCostBeforeDiscount into consignment interface

Rollout/Rollback

Revert

Testing

• existing test

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Note

Medium Risk
Adds new required fields to the shared Consignment TypeScript interface, which can break builds or consumers that construct consignments without populating them.

Overview
Extends the Consignment interface to include comparisonShippingCost and shippingCostBeforeDiscount so shipping totals can be represented both pre-discount and for comparison.

Updates consignments.mock.ts to populate these new fields in mock consignment objects.

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