Batch fetch current prices for multiple markets

Fetches current price/volume/liquidity for up to 300 (market_id, provider_id) pairs. Results are cached briefly. Every reference must contain a non-empty market identifier and a known integer provider_id; an invalid reference rejects the request. A provider with no batch-price support is skipped (its markets are simply absent from the response, not an error). Response is a flat object keyed by the requested market_id — no top-level wrapper.