Get Available Subcontractors

Get all companies that can be selected as subcontractors.

Args: assigned_only: If True, only return subcontractors currently assigned to vendors. If False (default), return all available options (active vendors + assigned subcontractors).

This includes:

1. All active vendors for the provided company_id (when assigned_only=False)
2. All companies that are already linked as subcontractors to any vendor