Use proper location for taxes when adding products via admin

[Konamiman]

All Submissions:

• Have you followed the WooCommerce Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

When the woocommerce_adjust_non_base_location_prices is set to always use the customer location to calculate taxes, and "I will enter prices inclusive of tax" setting is set, adding a product to an order as an admin via the order editor will calculate the wrong price, because no customer/order data is passed to wc_get_price_excluding_tax and thus the location of the currently logged in user (the shop admin) is used instead of the location of the customer.

This PR fixes that by changing WC_AJAX::maybe_add_order_item so that it passes the order to WC_Abstract_Order->add_product, which in turn passes it to wc_get_price_excluding_tax which then obtains the customer from the order and uses his location to calculate the taxes to be substracted from the price.

Closes #30231.

How to test the changes in this Pull Request:

1. Add the following snippet to your store: add_filter( 'woocommerce_adjust_non_base_location_prices', '__return_false' );
2. Make sure that you have the "Prices entered with tax" option set as "Yes, I will enter prices inclusive of tax" (WooCommerce - Settings - Tax - Tax options).
3. Let A be the country where your currently logged in user is based. Create an user based on a different country B.
4. Go to WooCommerce - Settings - Tax - Standard rates and set 20% for country A and 10% for country B.
5. Create a simple product, taxable with standard rates, with a price of 110.
6. Create an order from admin area, set U as the customer.
7. Add the product you just created to the order.

Without the fix, the price that will appear in the line item is 91,67 (110 minus 20% of taxes). With the fix, the price will be 100 (10% of taxes substracted this time).

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

Fix - Use proper location for taxes when adding products via admin

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.

[vedanshujain]

LGTM! Given that this is a very fundamental change, what do you think about adding more tests? I think we can add at least for:

1. add_product method when order is passed in $args with customer having different base address.
2. add_product method when order is passed in $args with customer having similar base address.
3. add_product method when order is passed in $args without customer.
4. ``add_product` method without order.

Maybe some more variations for when fileter is disabled/enabled as well?

[Konamiman]

@vedanshujain I think that since we are already testing that wc_get_price_excluding_tax is using the order it receives, it would be enough to add one test for verifying that add_product is passing the order to wc_get_price_excluding_tax and that the product is added with whatever price is returned by that function.

[vedanshujain]

LGTM!

[github-actions]

Hi @vedanshujain, thanks for merging this pull request. Please take a look at these follow-up tasks you may need to perform:

• Add the status: needs changelog label
• Add the status: needs testing instructions label