Fix incorrect quantity when product is already in cart

[martynmjones]

Changes proposed in this Pull Request:

Closes #391

Adjusts the utility to retrieve a product by its ID to prioritise the product data object over the cart object. This is to prevent the existing cart quantity from being sent with an add_to_cart event.

Additionally, the function to format product data is updated to pass along the quantity on the added_to_cart event.

Detailed test instructions:

1. Checkout fix/391-incorrect-product-quantity and build the extension
2. Go to the shop archive
3. Add a simple product to the cart multiple times
4. Refresh the page
5. Add the same product to the cart and confirm that the event sent has the correct quantity of 1
6. Go to the single product page
7. Set the quantity 4 and add it to cart
8. Confirm the quantity sent in the event is 4

Changelog entry

Fix - Incorrect quantity value when adding product to the cart that exists in the cart

[ianlin]

Thanks for fixing it @martynmjones. I tested both block and classic shops and it worked as expected. I'm approving this but just left a comment regarding the E2E test.

[ianlin]

❔ What do you think adding a page.reload() for the classic shop as well so it would match the testing instruction in the PR description?

[martynmjones]

Thanks for the review @ianlin 🙌

The error with the product quantity on the single page wasn't triggered with a page reload in the same way the other one was. When the added_to_cart hook runs the quantity wasn't being added to the product object so for the test we just need to click the Add to cart button and then confirm the quantity is now present in the event. Apologies if that wasn't clear from the testing instructions.