feat: Cypress test coverage enhancement for billing-address-validation

[akash-c-k]

Test Coverage

Test	What it verifies
Title renders	Confirms the checkout page loads successfully
Iframe loads	Ensures the payment element iframe is present
Card inputs hidden when billing pre-filled	No address fields appear when billing data is already provided
Payment succeeds with pre-filled billing	End-to-end payment flow works with pre-filled data
Address fields appear when billing missing	Dynamic field collection triggers when billing is absent
Validation errors on empty submit	Required fields block submission and show validation errors
Payment succeeds after filling fields	Full form completion → submit → success
State dropdown appears for US	Country selection dynamically adds state field
Postal code shown for US	Country-specific postal code field is rendered
State field hidden for Germany	Switching to DE removes the state dropdown