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. `markets_info` carries one record per market: its `symbol` and the venue's own `venue_size_increment`, the step the RFQ quantity must be a multiple of, so a client can size on the right grid as the user types. These are the venue instrument's grids, not the Paradex market's own increments: the two differ, and quantities are validated against the venue's. A quantity has to satisfy both grids, so combine them with their least common multiple rather than by taking the larger: the two coincide only when one divides the other, which holds for the powers of ten Paradex lists today but is not guaranteed of venue data. Neither grid is reliably the coarser one. Where they are coprime the combined step can exceed both (0.001 and 0.0025 give 0.005), which is correct: a quantity off either grid is rejected by one side or the other. The combined step and the minimum block size constrain different things and both always apply, so neither overrides the other. The step says which quantities are legal, the block size sets a floor, and the smallest quotable size is simply the first step at or above that floor. There is always one, so the two can never be mutually unsatisfiable. `venue_size_increment` is empty when the venue reports no usable grid for that market, or while an older service is still deployed. Read an empty one as "unknown" and do not fall back to the market's own increment alone, since a size legal on the order book can still be rejected by the venue. `POST /rfqs` validates the quantity against the venue grid and rejects an off-grid size before the RFQ is created, so that check remains the authority whatever a client sizes on. `markets` repeats the same symbols in the same order and is **deprecated**: it stays only until every client reads `markets_info`, and new integrations should ignore it. 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. `markets` 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. (`venue_size_increments` may legitimately be `{}` — an older service reports no grids at all.) Treat any failure as "unknown" and fall back to offering every market — the create remains authoritative.