feat: mark mandate_id as deprecated in VerifyResponse and MandateRevokedResponse
Add #[schema(deprecated)] to mandate_id fields in:
- VerifyResponse (payments.rs) — used in verify flow
- MandateRevokedResponse (mandates.rs) — used in mandate revocation
This ensures the OpenAPI-generated API reference correctly marks
these fields as deprecated for API consumers.
Refs: CON-23