[FIX] account: correctly reconcile refund from PoS

Current behavior:
In the PoS if you use 2 differents payment methods (cash and customer
account), and that the cash amount given by the client require you to
give some money back, the amount due on the customer account would be
incorrect on the invoice.

Steps to reproduce:
- Start a PoS session
- Add the 750$ desk to the order
- Go in the payment screen
- Add customer account with 300$
- Add cash with 460$
- Click on invoice
- The invoice show that the client need to pay 290$ which is not
  correct

Before the fix calling `_prepare_reconciliation_partials()`
with the sorted lines we had 2 credit lines (750€ and 10€)
and 1 debit line (460€).

Debit lines     |   Credit lines
460€            |   750€
                |   10€

The function would reconcile the 2 first lines and stop after that
because the only debit line was fully reconciled

After the fix we have this

Debit lines     |   Credit lines
460€            |   10€
                |   750€

After reconciling the 2 first lines we have this

Debit lines     |   Credit lines
450€            |   750€

And after reconciling the 2 last lines we have the correct amount left
to pay by the customer (300€)

opw-2857064

closes odoo/odoo#93910

Signed-off-by: William André (wan) <wan@odoo.com>