Update product validation logic for checkout

[ajello-meta]

Description

This PR updates product validation logic in Checkout.php. The WC()->cart->add_to_cart( $product_id, $quantity ) function does not throw an exception when the product ID (or quantity) is invalid. It also does not throw an exception when a product fails to be added to the cart. The updated logic (1) uses wc_get_product( $product_id ) to retrieve the product and validate it's existence and availability and (2) verifies that WC()->cart->add_to_cart( $product_id, $quantity ) returns true.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Syntax change (non-breaking change which fixes code modularity, linting or phpcs issues)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Test Plan

1. Go to http://test-site-i.local/fb-checkout?products=<ID>:<#>. Fill in the "ID" and/or "#" with invalid inputs.
2. Check Meta side logging to ensure that the error logs are visible.

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[github-actions]

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[facebook-github-bot]

@ajello-meta merged this pull request in 062f0a8.