Function fetch_deribit_expiries 

async fn fetch_deribit_expiries(underlyings: &[String]) -> Result<Vec<String>>

Fetch the current Deribit option term structure for the given underlyings.

Returns a sorted, deduplicated list of expiry timestamps in ISO 8601 format (e.g., “2026-04-11T08:00:00Z”) that Block Scholes will accept.