ViewSet for Message model.
primary key for the record as UUID
date and time at which a record was created
date and time at which a record was last updated
Serialize contacts.
Lightweight serializer for the user who sent a message.
Serialize message templates with dynamic body field inclusion.
Body fields (html_body, text_body, raw_body) are only included when explicitly requested via the ?bodies= query parameter or the body_fields keyword argument (for nested usage).
Allowed values: raw, html, text (comma-separated). Mapping: raw → raw_body, html → html_body, text → text_body.
When neither query param nor kwarg is provided, no body field is returned.
Return the STMSG headers of the message.