Update Afterpay/Clearpay BNPL branding to "Buy Now, Pay Later"

[cttsai-stripe]

Summary

• Update "Buy now or pay later" → "Buy Now, Pay Later" across all Afterpay/Clearpay/Cash App Afterpay English strings to match branding guidelines
• Fix AfterpayClearpayHeaderElementTest to expect the updated string

Motivation

Afterpay/Clearpay branding requires "Buy Now, Pay Later" (capitalized, comma-separated) instead of "Buy now or pay later". See RUN_MOBILESDK-5307.

Supersedes #12885. Aligned with iOS PR stripe/stripe-ios#6331.

Changes

English strings updated (in both values/ and values-en-rGB/):

• stripe_afterpay_clearpay_marketing: "Buy Now, Pay Later with <img/>" (payments-ui-core)
• stripe_afterpay_subtitle: "Buy Now, Pay Later with Afterpay" (paymentsheet)
• stripe_cashapp_afterpay_subtitle: "Buy Now, Pay Later with Cash App Afterpay" (paymentsheet)
• stripe_clearpay_subtitle: "Buy Now, Pay Later with Clearpay" (paymentsheet)

Translations: Not modified — translations are managed via Lokalize and will be updated in the next translation sync. The iOS PR takes a different approach (removing translations to fall back to English), but on Android the standard process is to let Lokalize handle it.

Testing

• AfterpayClearpayHeaderElementTest — updated assertion and verified passing
• AfterpayClearpayDefinitionTest — verified passing (uses resource resolution, not hardcoded strings)

Changelog

• [Changed] Updated Afterpay/Clearpay subtitle text from "Buy now or pay later" to "Buy Now, Pay Later" to match branding guidelines.

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