Function load 

pub async fn load(
    backend: &SessionBackend,
    sealer: &CookieSealer,
    cookie_value: &str,
) -> Result<Option<Vec<u8>>, StoreError>

Opens a session cookie value. Ok(None) = treat as unauthenticated (absent/expired/tampered/junk id); Err = store outage (503, never 401).