Fix coupon codes containing apostrophes

[mikejolley]

Submission Review Guidelines:

• I have followed the WooCommerce Contributing Guidelines and the WordPress Coding Standards.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have reviewed my code for security best practices.
• Following the above guidelines will result in quick merges and clear and detailed feedback when appropriate.

Changes proposed in this Pull Request:

Updates the wc_sanitize_coupon_code function to allow the ' character. I have included a additional test to cover this case, and I fixed a notice I encountered while testing which prevented me applying coupons (PHP8 notice due to a missing variable).

Closes #39481

How to test the changes in this Pull Request:

Using the WooCommerce Testing Instructions Guide, include your detailed testing instructions:

1. Add a coupon called test's. Any value is fine.
2. After saving the coupon, the coupon code test's should be visible in the coupon table. It should not be test\'s.
3. Go to cart on the frontend. Confirm the test's coupon can be applied to the cart. Place the order and ensure the coupon continues to function correctly.

Changelog entry

• Automatically create a changelog entry from the details below.

Significance

• Patch
• Minor
• Major

Type

• Fix - Fixes an existing bug
• Add - Adds functionality
• Update - Update existing functionality
• Dev - Development related task
• Tweak - A minor adjustment to the codebase
• Performance - Address performance issues
• Enhancement - Improvement to existing functionality

Message

Fixed coupon errors with coupon codes containing apostrophes.

Comment

[github-actions]

Test Results Summary

Commit SHA: c9605fd

Test 🧪	Passed ✅	Failed 🚨	Broken 🚧	Skipped ⏭️	Unknown ❔	Total 📊	Duration ⏱️
API Tests	259	0	0	2	0	261	0m 37s
E2E Tests	317	0	0	4	0	321	7m 8s

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To view the full API test report, click here.
To view the full E2E test report, click here.
To view all test reports, visit the WooCommerce Test Reports Dashboard.

[opr]

Looks good and works well, thanks!

Tests failing

[opr]

@mikejolley I can't get the tests working locally on this to debug, but I dismissed my review based on that.

[github-actions]

Hi @opr,

Apart from reviewing the code changes, please make sure to review the testing instructions as well.

You can follow this guide to find out what good testing instructions should look like:
https://github.com/woocommerce/woocommerce/wiki/Writing-high-quality-testing-instructions

[opr]

Looks good, tests OK. Clarified that wp_kses adds slashes unless you've got the unfiltered_html which is why we're removing them specifically for users without that capability.

Thanks Mike!