Fix itemsAdded to skip giftItems when creating the order items.

[nicholas-maestrello]

What problem is this solving?

The useQuote mutation was failing with CHK0023 "Cantidad de item inválida" when trying to add items to cart. Gift items were being included in the orderItems array sent to the price update API, but they shouldn't have price/quantity values set manually since they're automatically managed by promotions.
Also Add support for unitMultiplier on type QuoteItem and QuoteItemInput.

How to test it?

1. Create a quote that includes gift items from promotions
2. Use the quote via useQuote mutation
3. Verify items are added to cart without errors

[Workspace](Link goes here!)

Screenshots or example usage:

Before:

{"orderItems":[{"index":8},{"index":9}]}  // ❌ Gift items with undefined price/quantity

After:

// Gift items are filtered out from orderItems array ✅

Screen.Recording.2026-02-20.at.15.43.16.mov

Screen.Recording.2026-02-20.at.16.55.29.mov

Screen.Recording.2026-02-20.at.16.58.04.mov

Screen.Recording.2026-02-20.at.17.03.06.mov

Describe alternatives you've considered, if any.

Could have set price/quantity to 0 for gifts, but filtering them out is cleaner since gift prices are controlled by promotions.

Related to / Depends on

Fixes the root cause identified in useQuote error logs (CHK0023).

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