List VC IDs

Build unsigned XDR to list credential IDs in Vault.

POST /tx/prepare/list_vc_ids

• Body: { owner: string; vaultContractId?: string }

• Returns: { unsignedXdr: string }

• Validation errors: owner_required, bad_request (simulation)

• Source: api/testnet/src/controllers/txPrepareListVcIdsController.ts:5-20