Get VC

Build unsigned XDR to read a credential from Vault.

POST /tx/prepare/get_vc

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

• Returns: { unsignedXdr: string }

• Validation errors: owner_required, vcId_required, bad_request (simulation)

• Source: api/testnet/src/controllers/txPrepareGetVcController.ts:5-23