Function derive_ws_rpc_url 

pub(crate) fn derive_ws_rpc_url(
    ws_rpc_url: Option<&str>,
    engine_rpc_url: &str,
) -> Result<Url>

Returns the websocket RPC URL used for the persistence subscription.

Preference:

• If ws_rpc_url is provided, use it directly.
• Otherwise, derive a WS RPC URL from engine_rpc_url.

The persistence subscription endpoint (reth_subscribePersistedBlock) is exposed on the regular RPC server (WS port, usually 8546), not on the engine API port (usually 8551). Since we may only have the engine URL by default, we convert the scheme (http→ws, https→wss) and force the port to 8546.