[16.0] [FIX] sale_order_global_stock_route: fix nested SaleOrderLine Class in sale_order.py

[rrebollo]

This PR moves the SaleOrderLine class out of the SaleOrder class to make it a top-level definition. While nested classes are syntactically valid in Python and functional in Odoo, this change aligns with standard Odoo coding conventions for better readability, maintainability, and modularity. No functional changes are introduced.

Changes

• Un-nested SaleOrderLine from within SaleOrder.
• Ensured consistent 4-space indentation throughout the file.

Testing

• Verified module loads without errors in Odoo 16.
• Confirmed route inheritance and onchange behaviors remain intact.

This is also affecting v15,v17,v18.

@BinhexTeam