[IMP] uom: force factor to be 1 if reference unit
[IMP] uom: force factor to be 1 if reference unit

The _onchange_uom_type was already making sure it was the case but this
constraint ensure it is always the case.

Closes #25465

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo