parseSecureMessage abstract method
Parses a serialized Map into a SecureMessage instance.
Validates the structure (required fields: sender, receiver, data) and converts raw values (e.g., Base64 string → TransportableData) to proper types.
msg: Serialized secure message data (matches format in SecureMessage)
Returns: SecureMessage instance if parsing succeeds, null otherwise
Implementation
SecureMessage? parseSecureMessage(Mapping msg);