Get notifications, reactions and heard data for a message

MessageV6 differs from v5 in three ways: thread_id (the parent id, or the message id itself when it is not a reply) replaces parent_message_id; the audio rendition and its transcript, AI summary, language flags and time codes are nested under content, with the waveform as a compact base-36 string; and the requesting user's own state (heard_ms, label_ids, notify, notes, last_heard_update) is included where the read path carries it.