[FIX] point_of_sale: fix rounding logic

Use case specifically for round globally, but the justifications hold anyway
Have a session with one tax, multiple partners, and returns of freight (negative amounts)

When validating a pos.session, the computation of amls was a bit out of logic,
Namely, we grouped tax lines by :
1. their tax id (correct)
2. their partner_id (not wrong but limited)
3. their balance > 0 (wrong)

The justification for point 2 and 3 is two fold:
- Ultimately, the order is level of granularity that really makes sense
It's the order that is paid and therefore that has both legal and business meaning
and on which the amounts are retrieved anyway
- On the order, we litterally make the sum of tax amounts of each line, aggregating
credit and debit under the same variable
It is only logic to do the same for grouping the taxes at validation time

This commit only corrects the behavior for the rounding method 'globally'

OPW 1865700
closes #26203

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