This change reorganizes purchase order line preparation so it can be reused by approval-related purchasing flows without depending on stock operations. It should not change day-to-day behavior, but it makes the purchasing logic easier to share and maintain across Odoo modules.

**[MOV]** ***: _prepare_purchase_order_line**
> Move the method `_prepare_purchase_order_line` from 'stock.rule' to 'purchase.order.line'.
> The purpose is to reuse it in approvals_purchase.
> 
>task-2040710

This update renames an internal message-posting option so its expected value is clearer to developers. It should not change day-to-day user workflows, but it helps reduce future implementation mistakes across several business apps.