Function persist_client_draft 

pub fn persist_client_draft(
    client_id: &str,
    workspace_id: &str,
    session_id: &str,
    draft: &str,
) -> Result<()>

Store one viewer’s unsent draft.

An empty draft deletes the row rather than storing emptiness, so a viewer that cleared its composer stops occupying a row and stops being pruned later for something it no longer holds.