fix(e2e): configure the billing postal label

[kieran-osgood-shopify]

Stack Context

This PR builds on merged #560, which introduced the shared hardcoded-buyer checkout and billing-address flow.

What?

Read the billing postal-field label from the checkout fixture instead of hardcoding Postal code in the shared flow.

Why?

Checkout localizes the field by market: Canadian addresses show Postal code, while United States addresses show ZIP code. Keeping the label beside the fixture lets each checkout journey match the form produced by its address.

How to test

• Run the shared hardcoded-buyer checkout journey.
• ./scripts/test_ruby

[kieran-osgood-shopify]

• fix(e2e): configure the billing postal label #673 👈 (View in Graphite)
• main

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[bitrise]

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Checkout Kit E2E results

Status	Suite	Target	Platform	OS version tag	Device
✅	react-native-ios	react-native	ios	latest	iPhone 15 iOS 27 Beta
✅	react-native-android	react-native	android	latest	Google Pixel 9 Android 17.0
✅	kotlin-android	kotlin	android	latest	Google Pixel 9 Android 17.0
✅	swift-ios	swift	ios	latest	iPhone 15 iOS 27 Beta