Add configurable futures roll date

[scarab-systems]

Summary

• Adds DataMappingMode.TradingDaysBeforeExpiry for continuous futures that need to roll before the normal last-trading-day mapping date.
• Threads a tradeable-day roll offset and optional contract month cycle through continuous future subscriptions, universe settings, history requests, mapping events, and the Python wrapper.
• Reuses existing LastTradingDay map-file rows for the new mapping mode and applies the optional contract month cycle when walking continuous future contract depth.

Motivation

Fixes #9440.

The issue describes two related continuous futures needs:

• rolling a configurable number of trading days before the normal last-trading-day roll;
• restricting held futures contracts to selected contract months, for example skipping weak open-interest months.

Testing

• dotnet build Tests/QuantConnect.Tests.csproj --no-restore -clp:ErrorsOnly --verbosity quiet

Added focused tests for:

• TradingDaysBeforeExpiry reusing LastTradingDay map-file rows;
• contract-depth walking with a selected contract month cycle;
• skipping a current mapped contract when its month is outside the selected cycle;
• adding tradeable days across closed exchange dates.

I was able to build the test project locally. Direct local NUnit execution in my Linux container aborted during global Python/test-host initialization before the selected tests ran, so CI should be the authoritative full test run.