Add support for ECE Payment buttons on Pay for Order page

[james-allan]

Closes #3411

Changes proposed in this Pull Request:

This PR adds support for processing Express payment button payments via the Pay for Order page.

^{Google Pay and Apple Pay buttons on the Pay for Order}

Testing instructions

SETUP

1. Enable Apple Pay / Google Pay in your Stripe plugin settings.
2. Enable the ECE feature flag.
   • Update the _wcstripe_feature_ece option. See WC_Stripe_Feature_Flags::is_stripe_ece_enabled()

PAYMENTS

1. Go to WooCommerce → Orders on your WP Admin dashboard.
2. Click the Add order button.
3. Add your current user as the customer.
4. Add any items.
5. Calculate totals.
6. Create the order.
7. In the order details section click the "Customer payment page" link. (Screenshot)
8. On the following page note that the Apple Pay and/or Google Pay buttons are shown.
9. Choose either Google Pay or Apple Pay
10. Confirm the modal that pops up has correct totals etc.

Order totals	Apple Pay screens

13. Complete the payment.
14. Confirm you are redirected to the order received page.
15. In your Stripe Dashboard confirm the payment is completed and the total matches the order total.

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• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)

[james-allan]

This change is just a blank space at the end of the line which is being removed.

[mattallan]

Weird that GH isn't showing the whitespace being removed!

[james-allan]

This fixes an issue on all shortcode powered pages and follows the same approach WooPayments took in Automattic/woocommerce-payments#9094 and discussed with Stripe internallly here: p1720833500937719-slack-C9976E5MJ

PAY FOR ORDER

^{Gap on the right hand side of the payment elements.}

CHECKOUT

CART

[mattallan]

Really well put together PR @james-allan ! 💯

I've performed code review and the changes look good.
I've also tested the following flows/cases:

• Paying for an order with virtual/downloadable products
• Paying for an order with various shipping & tax totals
• Paying for an order with/without prices includes tax
• Paying for an order with discounts applied
• Paying for an order with multiple products
• Paying for an order with mix of variable/simple products
• Confirm orders have correct payment method title
• Order under minimum amount (0.20). Confirmed buttons don’t show.
• Test Apple Pay and Google Pay

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Something I noticed is the Pay for Order doesn't add the quantity label like we do for the product page.

I.e.

Pay for Order	Product Page

This is a very minor detail and I noticed WooPayments does the same thing so happy to run with it as is.

[james-allan]

Something I noticed is the Pay for Order doesn't add the quantity label like we do for the product page.

oh neat. I'll file a separate issue for this because I think that's a good enhancement.