[Futures] Allow reducing leveraged targets during delta buying-power liquidation

[aiSynergy37]

Description

• allows FutureMarginModel leveraged target requests (|target| > 1) only when they reduce existing exposure
• keeps the existing exception behavior for leveraged requests that increase exposure
• adds a regression test for the delta-buying-power path used by margin liquidation

Closes #5604.

Testing

• dotnet build Tests/QuantConnect.Tests.csproj --no-restore -v minimal
• dotnet test Tests/QuantConnect.Tests.csproj --filter FullyQualifiedName~FutureMarginBuyingPowerModelTests.GetMaximumOrderQuantityForDeltaBuyingPower_AllowsReducingLeveragedTarget --no-build -v normal -m:1 (test host crashes on this machine with Python.NET GIL finalizer error before test completion)