[FIX] sale_stock, purchase_stock: fix conversion qty received

Rounding in purchase order is 'UP' which is causing issue when doing
receipt in a different uom than the purchase. Rounding should be
'HALF-UP'

Added test in purchase and sales to ensure this issue is detected in the
 future.

closes odoo/odoo#75975

X-original-commit: 3f6f5868
Signed-off-by: William Henrotin <Whenrow@users.noreply.github.com>