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• Have you followed the WooCommerce Contributing guideline?
• Does your code follow the WordPress' coding standards?
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Changes proposed in this Pull Request:

Closes #28166.

This PR adds the storage of WC_Coupon data to Order Item Meta when Orders are created outside the Cart/Checkout context, like through the REST API or wp-admin Dashboard.

Without these changes, order coupon line item data can vary depending on the context the order was created in:

When you place an order via checkout (i.e. normal woocommerce flow) - coupon_data is added to the order's coupon_lines metadata here: https://github.com/woocommerce/woocommerce/blob/trunk/includes/class-wc-checkout.php#L626-L629

But when you add a coupon to an order programmatically, via apply_coupon in our case (or create the order programmaticall in full), that coupon_data metadata is not added at all.

How to test the changes in this Pull Request:

1. Ensure you have a coupon you can apply to a new order
2. Ensure you have a product you can add to the order
3. Create a new order via the REST API (POST https://example.com/wp-json/wc/v3/orders):

{
    "status": "on-hold",
    "currency": "USD",
    "payment_method": "cod",
    "payment_method_title": "Cash on Delivery",
    "line_items": [
        {
            "product_id": 5,
            "quantity": 1
        }
    ],
    "coupon_lines": [
        {
            "code": "couponcode"
        }
    ]
}

4. Verify that the resulting order response has meta_data in the coupon_lines that contains data like:

"meta_data": [
        {
          "id": 2016,
          "key": "coupon_data",
          "value": {
            "id": 130,
            "code": "couponcode",
            "amount": "7",
            "date_created": {
              "date": "2020-10-30 11:06:20.000000",
              "timezone_type": 3,
              "timezone": "Europe/Madrid"
            },
            "date_modified": {
              "date": "2020-10-30 11:07:19.000000",
              "timezone_type": 3,
              "timezone": "Europe/Madrid"
            },
            "date_expires": null,
            "discount_type": "percent",
            "description": "",
            "usage_count": 2,
            "individual_use": false,
            "product_ids": [],
            "excluded_product_ids": [],
            "usage_limit": 0,
            "usage_limit_per_user": 0,
            "limit_usage_to_x_items": null,
            "free_shipping": false,
            "product_categories": [],
            "excluded_product_categories": [],
            "exclude_sale_items": false,
            "minimum_amount": "",
            "maximum_amount": "",
            "email_restrictions": [],
            "virtual": false,
            "meta_data": []
          }
        }
      ]

5. Create a new order through the wp-admin dashboard (WooCommerce > Orders > Add Order)
6. Add a product ("Add item(s)")
7. Add a coupon ("Apply coupon")
8. Retrieve the order from the REST API (GET https://example.com/wp-json/wc/v3/orders/[ORDER_ID])
9. Verify the coupon_lines contains meta_data like the previous example

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.

Fix - Ensure coupon data is stored consistently when created outside of frontend checkout.

FOR PR REVIEWER ONLY:

• I have reviewed that everything is sanitized/escaped appropriately for any SQL or XSS injection possibilities. I made sure Linting is not ignored or disabled.