Test/apple pay google pay checkout

[yashranaway]

Add system tests for Apple Pay/Google Pay checkout

Fixes #2173

Summary

Adds system tests for Apple Pay/Google Pay checkout flows with mocked Stripe payment request API.

Changes

Added
spec/requests/checkout/apple_pay_google_pay_spec.rb

Verified

Payment method selection works correctly
No duplicate Pay buttons appear
Error handling when payment unavailable

[yashranaway]

@binary-koan i have reponed #2182 with the fixes

[yashranaway]

I updated the mock to support simulating a payment failure, so the tests now explicitly click the button and assert that the 'Sorry, something went wrong' error message is displayed. LMK if anything is wrong, thankyou

[binary-koan]

Just tried this out and it doesn't work locally because the checkout is a full page load that wipes out the mock value. Could you check if there's a workaround?

[yashranaway]

@binary-koan

I moved the mock injection to after add_to_cart so it runs on the checkout page after navigation completes.
I can't test locally due to webpack build errors affecting all system specs - could you please verify this works on your end?

[EmCousin]

@binary-koan I'm assigning this to you since you're already involved in it - it also helps me track which PR still await for review. However let me know if you prefer a different way of tracking PRs!