go-hyperliquid's NewExchange auto-fetches meta/spotMeta/perpDexs and
panics if any of those API calls fail (NewInfo: panic(err)). The quick
trade flow constructs a probe trader inside POST /api/traders, so any
transient Hyperliquid API hiccup crashed the request with a recovered
panic and a bare 500. Wrap the constructor in initExchangeClient, which
converts the panic into an error that surfaces through the existing
exchange-probe validation path as an honest, retryable message.
