Builds and returns the rendered email content for a specific article (email-article) or lift letter (email-promo / email-renewal). Combines the chosen template (template_id) with the resource and publication context, applies macro replacement and video thumbnails, and returns the merged HTML in content plus the resource's subject and preview_line. This is the standard step before calling POST /messages (createMessage): take the returned content, subject, and preview_line and pass them straight into the create payload. The create endpoint does NOT re-render content from template_id — the content you submit is what gets sent. This endpoint is for resource-backed message types only. Free-style (email-free-style) messages do NOT use this endpoint — fetch the template directly via GET /templates/{id} (retrieveTemplate) instead, strip the {$sys_email.content} (or {$sys_email_content}) placeholder, write the body in its place, and submit that as content on POST /messages.