Returns a single broadcast message by UUID, scoped to the authenticated account. The body comes back as three fields: html_content (the visual-editor HTML), raw_html_content (custom raw HTML supplied by the user), and use_raw_html_content (when true, raw_html_content is the body that sends; otherwise html_content sends -- and if use_raw_html_content is true but raw_html_content is empty, the system falls back to html_content).