List markets available for RFQ

Returns the market symbols currently available for RFQ. Not every listed Paradex market can be quoted as an RFQ: the venue quotes only the instruments it lists for the Paradex venue, and a create naming any other market is rejected with a validation error. This endpoint is that set, so a client can stop offering markets a create would reject on their name instead of discovering the difference after the user has already sized a structure. Membership is necessary but not sufficient. A create additionally checks that a hedge leg's price sits on the instrument's tick and that the strategy quantity meets its minimum block size, so a market listed here can still be rejected on those grounds. The set is served from a periodically refreshed snapshot, so a market listed by the venue moments ago may take up to one refresh interval to appear here. The response is never an empty list: a caller cannot tell "the venue quotes nothing" from "this read is unavailable", so a degraded or empty snapshot is reported as `503` instead. Treat any failure as "unknown" and fall back to offering every market — the create remains authoritative.