[FIX] account: reconciliation models: make auto_reconcile rules work properly with write-offs

- Create an invoice for 100
- Create a bank statement for 95
- Setup your reconciliation model so that it will match them and create a write-off for the remaining 5. Make it auto_reconciled.

=> Doing that, we expect the bank journal move to be created like this:

- 100 on payable/receivable, reconciled with the invoice
- 95 on the bank account
- 5 on the write-off account

This wasn't the case. Instead, Odoo created the following (correct from an accounting point of view, but weird):

- 95 on payable/receivable, reconciled with the invoice
- 5 on payable/receivable, not reconciled
- 95 on the bank account
- 5 on the write-off account

=> So an open balance of 5 was actually created. This was weird, and also caused an additional bug, as only the first payable/receivable line was reconciled with the invoice, making it only partially paid.

This commit restores the expected behavior.



[FIX] account: reconcile models: create write-offs properly when matching a statement line with no partner

Before that, write-offs were simply never created in this case ; only an open balance. This was (obviously) wrong.

Also, the partner set on the statement line was not modified, and kept empty. We fix that so that, when a statement line without is reconciled with lines sharing the same partner, this partner is also assigned to the statement line.

Task 2423231