Signs a message using the SVM wallet identified by the supplied walletMetadata. The SDK is stateless — every call requires walletMetadata. If the wallet was created with backUpToDynamic: true, supply the password so the SDK can recover shares from backup when externalServerKeyShares is omitted.
When you pass externalServerKeyShares (the caller-supplied path), walletMetadata.externalServerKeySharesBackupInfo must also be present — signMessage 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.