Refactor SetHoldings to return a List<OrderTicket>

[JosueNina]

Description

Changed the SetHoldings methods to return a List instead of void

Related Issue

Closes #8547

Motivation and Context

This change ensures that SetHoldings returns executed order tickets, making it easier to track orders.

Requires Documentation Change

No.

How Has This Been Tested?

Tested with unit tests using multiple test cases for various scenarios to ensure the correct return of order tickets in SetHoldings.

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 performance. Please add associated performance test and results)
• 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 existing tests passed.
• My branch follows the naming convention bug-<issue#>-<description> or feature-<issue#>-<description>