Function extract_inner_error 

pub fn extract_inner_error(
    response: &HyperliquidExchangeResponse,
) -> Option<String>

Extracts per-item error from a successful Hyperliquid exchange response.

When the top-level status is “ok”, individual items in the statuses array may still contain errors. Returns the first error found, or None if all items succeeded or the response cannot be parsed.