Function handle_close_session 

fn handle_close_session<H, C>(
    service: &mut CryptoService<H, C>,
    tx_buf: &mut [u8],
) -> usize
where
    H: AteccHal,
    C: Clock,
    H::Error: Debug,

Synchronous handler: closes the PIN session in the host-side state only, no chip interaction. Always succeeds (idempotent).