Add logging for coupon in checkout

[ajello-meta]

Description

This PR adds logging for coupon codes in Checkout.php. The WC()->cart->apply_coupon( $coupon_code ) function does not throw an exception when the coupon is invalid. The updated logic adds a conditional check that verifies the coupon is correctly applied to the cart, or logs the error if not correctly applied to the cart.

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>:<#>&coupon=<code>. Fill in the "ID" and/or "#" with valid inputs. Fill in "code" with an invalid input.
2. Check Meta side logging to ensure that the error logs are visible for flow_name=checkout and flow_step=apply_coupon.

[facebook-github-bot]

@ajello-meta has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[github-actions]

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Built at: 2025-05-29T23:51:07.648Z
Commit: 1f37eb8
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[facebook-github-bot]

@ajello-meta merged this pull request in 36cbefd.