Exports the private key for the SVM wallet identified by the supplied walletMetadata. Returns a base58-encoded Solana keypair string.
When you pass externalServerKeyShares (the caller-supplied path), walletMetadata.externalServerKeySharesBackupInfo must also be present — exportPrivateKey throws if shares are supplied but backup metadata is missing. The full walletMetadata returned from createWalletAccount / importPrivateKey already includes it; identity-only metadata from fetchWalletMetadata will be rejected.