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:

During checkout validation, there are points where we specifically rely on various $_POST superglobal values being set, which made reuse of our validation functionality difficult. This change tidies this up.

Closes #35328.

How to test the changes in this Pull Request:

Checkout validation should continue to work as it did previously. More specifically, we should ensure that we are still validating if the custom has agreed to the shop's terms and conditions (since the change touches that check):

1. Ensure you have designated a terms and conditions page (via WooCommerce ▸ Settings ▸ Advanced ▸ Page Setup):

2. As a customer, add a product to your cart and attempt to checkout. In the first instance, populate the checkout form correctly but avoid checking the box that indicates you agree with the terms and conditions:

3. Consequently, on trying to checkout, you should see a warning as follows:

4. Rinse and repeat; except agree to the terms and conditions. If all other fields are correctly populated, you should be able to checkout successfully.
5. Repeat again, but try to supply invalid values to other checkout fields (for example, you might provide an invalid postal code) and confirm validation successfully prevents this.

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 created a changelog file for each project being changed, ie pnpm --filter=<project> changelog add?

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.