Root cause: PlaceMarketOrder manually calculated limitPx=mid*0.5 (sell)
then passed &limitPx to SDK's MarketOpen. SDK treated this as reference
price and applied slippage AGAIN (0.95x), resulting in limitPx=mid*0.475.
Notional value fell below HL's 0 minimum → error.
Fix: pass nil instead of &limitPx, letting SDK get mid price and
apply standard 5% slippage itself.