This fixes an issue in the manufacturing planning forecast details where purchase order quantities could be handled incorrectly. The change uses the quantity value that is already in the right unit, helping ensure planning figures are accurate without unnecessary conversion.

There was an error due to this PR:
https://github.com/odoo/odoo/pull/186250
The field in the wizard should have been renamed, but since
`product_uom_qty` is already in the correct unit, we can
simply do the sum without converting.