Buying power validation for combo orders updates

Expected Behavior

Buying power validation is done for combo orders, so that updates are rejected if margin is not enough for the requested new order settings.

Actual Behavior

Buying power validation is done only for individual orders and skipped for combo orders.
See https://github.com/QuantConnect/Lean/pull/8553.

Potential Solution

OrderTicket.Update might have an overload to take a collection of updates for each leg of a combo, and updates request can be queued in a batch together. That way the brokerage transaction handler can validate the buying power for the whole combo.

Reproducing the Problem

See https://github.com/QuantConnect/Lean/pull/8553 and comment out if (order.GroupOrderManager == null) in BrokerageTransactionHandler.HandleUpdateOrderRequest() to allow updates to combo orders legs. The regression algorithms that update combo orders start failing.

System Information

Windows 11

Checklist

• I have completely filled out this template
• I have confirmed that this issue exists on the current master branch
• I have confirmed that this is not a duplicate issue by searching issues
• I have provided detailed steps to reproduce the issue

Start with BrokerageTransactionHandler.HandleUpdateOrderRequest() and review the related changes in pull request 8553. Reproduce the issue using the regression algorithms that update combo orders, then trace OrderTicket.Update and combo-leg handling. Done means combo-order updates are rejected when the requested settings exceed available buying power, while valid updates continue to work.