Validate buying power for combo order updates

[mameikagou]

Description

Stage combo legs on isolated copies, apply the proposed update to the staged group, and validate buying power before mutating the live order. Rejected updates now leave the combo group unchanged and never reach the brokerage.

Related Issue

Fixes #8567

Motivation and Context

Combo order updates skipped buying-power validation, allowing updates that exceeded available margin.

Requires Documentation Change

No.

How Has This Been Tested?

Built QuantConnect.Tests with .NET 10 and ran seven focused brokerage transaction-handler tests in a bounded Docker environment, including the new accepted and rejected combo-update cases. All seven passed.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Refactor (non-breaking change which improves implementation)
• Performance (non-breaking change which improves implementation)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Non-functional change (xml comments/documentation/etc)

Checklist:

• My code follows the code style of this project.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and focused existing tests passed.
• My branch follows the naming convention bug-<issue#>-<description> or feature-<issue#>-<description>