Update Stripe retry_failed_payment api access to payment intent

[mguidetti]

Pull Request

Summary:
Fix retry_failed_payment for Stripe API versions > 2025-03-31

Related Issue:

Description:
Stripe API 2025-03-31 added support for multiple payments to invoices and removed the payment_intent field from the Invoice object in favor of the payments collection (https://docs.stripe.com/changelog/basil/2025-03-31/add-support-for-multiple-partial-payments-on-invoices)

retry_failed_payment was referencing invoice.payment_intent, which raises a NoMethodError on Stripe API versions >= 2025-03-31.

This PR changes retry_failed_payment to access the payment intent through the payments collection instead. It selects the first payment using .first, so this doesn't actually support multiple payments like the API does, but perhaps this is fine for this gem's current purposes?

Testing:

I didn't find any existing tests for this method and was unsure the best way to test it.

Screenshots (if applicable):

Checklist:

• Code follows the project's coding standards
• Tests have been added or updated to cover the changes
• Documentation has been updated (if applicable)
• All existing tests pass
• Conforms to the contributing guidelines

Additional Notes: