Function fix_session_memento 

pub fn fix_session_memento(
    db_path: &Path,
    valid_session_ids: &HashSet<String>,
    preferred_session_id: Option<&str>,
) -> Result<bool>

Fix the memento so it points to a session that actually exists. If the current memento references a deleted/non-existent session, update it to the most recently active valid session. Returns true if the memento was changed.