createInstantMessage abstract method
Creates an instant message from envelope (header) and content (body).
Combines routing metadata (envelope) with message payload (content) to form a complete, unencrypted instant message.
head is the message envelope (routing metadata: sender/receiver/time).
body is the message content (payload: text, file, command, etc.).
Returns a complete InstantMessage instance.
Implementation
InstantMessage createInstantMessage(Envelope head, Content body);