[Gift Cards] Fix decoding error when gift card amount has non-integer value

[rachelmcr]

Closes: #9793

Description

This PR fixes a decoding error for stores using the Gift Cards extension, if the gift card in an order has an amount that is a non-integer. This is because the amount property for OrderGiftCard was mistakenly given an Int64 type, so it couldn't handle gift card amounts with decimal values. This is resolved by updating the amount to use a Double type to handle those non-integer amounts.

Changes

• Updates the amount property in OrderGiftCard in the Networking layer to use Double type.
• Updates the amount attribute in the OrderGiftCard entity in the Storage layer use Double type. This includes a new Core Data model and migration test for the change.

Testing instructions

Prerequisites

1. Install and activate the Gift Cards extension.
2. Create a gift card product.
3. Create an order for a gift card product, to create a working gift card with a code you can apply to an order.
4. Create an order with a non-integer order total, and apply the gift card to pay for the order.

To test

1. Build and run the app.
2. Open the Orders tab and confirm the order list loads.
3. Select the order from step 4 above and confirm the order details Payment and Gift Card sections show a gift card with non-integer amount applied to the order.

Screenshots

Before	After - Order list	After - Order details

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• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[wpmobilebot]

You can test the changes from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr9795-1b0c8b1 on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[peril-woocommerce]

	Warnings
⚠️	PR has more than 500 lines of code changing. Consider splitting into smaller PRs if possible.

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