[kotlin-sdk] Decrypt-on-fetch for encrypted wallet-contract documents #4087

Priority: P0 — hard blocker, pairs with the encrypted-document create issue; no app-side workaround. Implementation will be driven by @bfoss765.

queries/Documents.search/fetch return plaintext documents. The wallet's TxMetadata fetch/merge loop needs the legacy equivalent of getTxMetaData(sinceTime, encryptionKey): fetch the wallet contract's documents since a timestamp and decrypt them with the identity's encryption key, so memo/taxCategory/exchangeRate/service/giftCard fields can be reconciled into the local store.

Ask: decrypt-on-fetch (or a fetch variant taking/deriving the identity encryption key) for encrypted wallet-contract documents.

Context: dash-wallet migration plan Phase 3h step 3 (dashpay/dash-wallet#1507). cc @QuantumExplorer