createInstantMessage abstract method
Creates a new InstantMessage from envelope and plaintext content.
Combines routing metadata (envelope) with unencrypted content to form a complete instant message (plaintext stage).
head: Message envelope (routing metadata, cannot be null)
body: Plaintext content (message payload, cannot be null)
Returns: New InstantMessage instance
Implementation
InstantMessage createInstantMessage(Envelope head, Content body);