docs(checkout): CHECKOUT-9857 Document storefront checkout shipping comparison fields (#1287)

<!-- Ticket number or summary of work -->
# [CHECKOUT-9857]


## What changed?
Updates the Storefront Checkouts spec (reference/checkouts.sf.yml) for
the Get a Checkout response.

- checkout_Full: clarify shippingCostBeforeDiscount (total shipping
before discounts) and document comparisonShippingCost (total shipping
after automatic promotions).
- consignment_Full: add shippingCostBeforeDiscount and
comparisonShippingCost with the same meanings per consignment.
- CheckoutResp example: include the new fields.

## Release notes draft
* The [Storefront Checkout
API](https://developer.bigcommerce.com/docs/rest-storefront/checkouts#get-a-checkout)
response now documents shipping comparison fields so you can show
shoppers the right “original” vs. promotional shipping amounts. Checkout
and each consignment include **shipping cost before discount** and
**shipping cost after automatic promotions**, which makes it easier to
display fair pricing and explain savings at checkout.

## Anything else?
<img width="1678" height="2062" alt="image"
src="https://github.com/user-attachments/assets/bbab3739-17d5-4702-ae64-fc6079a95f9a"
/>

ping @bigcommerce/team-checkout 


[CHECKOUT-9857]:
https://bigcommercecloud.atlassian.net/browse/CHECKOUT-9857?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: bc-mackxu

reference/checkouts.sf.yml

@@ -897,6 +897,8 @@ components:
               cost: 69.94
               transitTime: 1 business day
             shippingCost: 69.94
+            shippingCostBeforeDiscount: 69.94
+            comparisonShippingCost: 64.94
         coupons:
           - code: S2549JM0Y
             couponType: 2
@@ -941,6 +943,7 @@ components:
         payments: [{}]
         promotions: []
         shippingCostBeforeDiscount: 69.94
+        comparisonShippingCost: 64.94
         shippingCostTotal: 69.94
         shouldExecuteSpamCheck: false
         subtotal: 117.93
@@ -1066,7 +1069,12 @@ components:
                 description: Text displayed on the storefront for the promotion.
         shippingCostBeforeDiscount:
           type: number
-          description: The shipping cost before discounts are applied.
+          description: Total shipping cost for the checkout before discounts are applied.
+          format: float
+        comparisonShippingCost:
+          type: number
+          description: Total shipping cost for the checkout after automatic promotions are applied.
+          format: float
         shippingCostTotal:
           type: number
           description: Shipping cost before any discounts are applied.
@@ -1447,6 +1455,14 @@ components:
           type: number
           description: The shipping cost for this consignment.
           format: double
+        shippingCostBeforeDiscount:
+          type: number
+          description: This consignment's shipping cost before discounts are applied.
+          format: double
+        comparisonShippingCost:
+          type: number
+          description: This consignment's shipping cost after automatic promotions are applied.
+          format: double
         lineItemIds:
           type: array
           description: ''