Issue #3610814: orchestra_payment: extend kessai's PaymentStorageSchema instead of re-declaring its index

Follow-up to kessai #3610757 (kessai MR !13 (merged)), which makes kessai's PaymentStorageSchema extensible.

orchestra_payment previously extended SqlContentEntityStorageSchema directly and re-declared kessai's kessai_payment__subject_kind index by hand, only because kessai's class was final. This changes it to extend \Drupal\kessai\PaymentStorageSchema and add its (orchestra_token, kind, state) index through the indexes() seam (parent::indexes() + its own), so the base subject index is inherited and no longer duplicated or kept in step by hand. The getEntitySchema() override is removed.

PaymentIndexTest keeps its assertions (both indexes must survive the handler swap); only its docblock is updated to the new mechanism.

Blocked by: kessai #3610757. Until that lands in kessai, dev kessai still ships the final class, so CI here fails on the extend. Re-run the pipeline once kessai !13 (merged) is merged, then raise the kessai dependency constraint accordingly.