[FIX] purchase_ux: add product_id and selected_seller_id to name compute depends

[les-adhoc]

Summary

• In v19, _compute_price_unit_and_date_planned_and_name (which sets the name/Description field on PO lines) lacked product_id and selected_seller_id in its @api.depends.
• OWL did not know to re-render the Description column when the product changed, leaving it blank until the next repaint (e.g. Alt+Tab).
• The backend was already computing the correct value via the legacy @api.onchange('product_id'), but the list widget never received the signal to update.
• Fix: extend the @api.depends in the purchase_ux override with product_id and selected_seller_id so OWL triggers the recompute and re-renders the field immediately on product selection.

Test plan

• Open a draft PO, add a new line and select a product that has a vendor with a reference code — Description should auto-fill immediately (no save or focus-change required).
• Confirm that changing the product on an existing draft line updates the Description correctly.
• Confirm that changing the product on a confirmed PO line does not update Description (existing purchase_ux guard: state not in ['purchase', 'done']).
• Confirm price and date_planned still compute correctly on product selection.

[roboadhoc]

[mav-adhoc]

@roboadhoc r+ nobump