Add futures GetOpenOrdersAsync (list-open-order endpoint)

[Da-Teach]

Summary

The USDT-M futures trading client (IXTRestClientFuturesApiTrading) currently exposes only GetOrdersAsync, which maps to the order-history endpoint GET /future/trade/v1/order/list (filtered by state + startTime/endTime + pagination).

XT also offers a dedicated open-orders endpoint, GET /future/trade/v1/order/list-open-order, which returns only NEW / PARTIALLY_FILLED orders, takes an optional symbol (all markets when omitted), and has no time window or pagination. It's the futures analogue of the existing SpotApi.Trading.GetOpenOrdersAsync — but there was no futures wrapper for it.

This PR adds FuturesApi.Trading.GetOpenOrdersAsync(string? symbol = null, CancellationToken ct = default).

Without it, callers that need a complete snapshot of currently-open futures orders have to fall back to the history endpoint and guess at its default time window — which risks missing an old resting order.

Changes

• IXTRestClientFuturesApiTrading.GetOpenOrdersAsync (interface + XML docs)
• XTRestClientFuturesApiTrading.GetOpenOrdersAsync (implementation)

The implementation mirrors the existing spot GetOpenOrdersAsync (returns XTFuturesOrder[], single GET, optional lowercased symbol) and the futures rate-limit guard used by the sibling GetOrdersAsync.

public async Task<WebCallResult<XTFuturesOrder[]>> GetOpenOrdersAsync(string? symbol = null, CancellationToken ct = default)
{
    var parameters = new ParameterCollection();
    parameters.AddOptional("symbol", symbol?.ToLowerInvariant());
    var request = _definitions.GetOrCreate(HttpMethod.Get, "/future/trade/v1/order/list-open-order", XTExchange.RateLimiter.RestFutures, 1, true, limitGuard: new SingleLimitGuard(200, TimeSpan.FromSeconds(1), RateLimitWindowType.Sliding));
    var result = await _baseClient.SendAsync<XTFuturesOrder[]>(request, parameters, ct).ConfigureAwait(false);
    return result;
}

Builds clean against the current main. Note: I matched HttpMethod.Get to the sibling order/list futures query and the spot open-order endpoint; if XT actually expects POST here, happy to switch.