[FIX] purchase: no reset custom description in PO if qt change

Step to reproduce:
	Install purchase
	Create a purchase order
	Set a vendo and a product that has this vendor in its
	vendor list
	Set a product name in the line of the vendor in the product
	purchase section (you will need to add the field)
	Change the description of the product
	Change the quantity of the product

Expected behavior:
The description stay the custom input you just set

Current behavior:
The description is reset to its default value

Explanation:
When changing the quantity the vendor from the product vendors can change.
Then its vendor product name and code can change and thus the default
description in the purchase order. To solve that we need to differentiate
a custom description from a default one and only update the descritpion
when the quantity changes if the descritpion is a default one (and not a
sutom one)

opw-2827667

closes odoo/odoo#90764

X-original-commit: d5408b80
Signed-off-by: Arnold Moyaux <arm@odoo.com>
Signed-off-by: Fockedey Martin (mafo) <mafo@odoo.com>