[FIX] point_of_sale: mix of sale and refund

- Set rounding policy to 'Round Globally'
- Create a Tax T
  Amount: 21.0%
  Different tax tags for invoice and credit notes
  Set any account Repartition Line
  Advanced Options => Included in Price
- Create a Product P1
  Sales Price: 11.8
  Customer Taxes: T
- Create a Product P2
  Sales Price: 15.3
  Customer Taxes: T
- Create a new POS Session
- Create a POS order:
  P1, Quantity: -6, Discount: 5%
  P2, Quantity: 6, Discount: 5%
- Validate and close the session

A difference of 0.01 prevents from closing the session.

The error arises because the tax tags are different for an invoice and a
refund. In the following:
https://github.com/odoo/odoo/blob/1a9e147639012cc847099e69fc964f3669a36cbb/addons/point_of_sale/models/pos_session.py#L394

Both lines have different `tag_ids`, therefore their amount is not
summed when closing the session. On the other hand, the POS UI is not
aware of the `tag_ids`, so it sums then round the amounts.

The situation is quite unusual: from SO or Invoices, it is possible to
create either an invoice or a refund, but not a mixed object as the POS
is doing. Moreover, if a user invoices the POS order, there is no
problem to close the session since a regular invoice is created and tax
amounts are summed.

We consider a refund if all lines have negative quantities. We only do
it in 'Round Globally' configuration to avoid impacting non-problematic
use cases.

opw-2254211

closes odoo/odoo#52704

X-original-commit: 5f25be8eef9a9efc18a995f2c6405d08a657ac96
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/point_of_sale/models/pos_session.py

@@ -680,7 +680,14 @@ class PosSession(models.Model):
                     .filtered(lambda t: t.company_id.id == order_line.order_id.company_id.id)
         sign = -1 if order_line.qty >= 0 else 1
         price = sign * order_line.price_unit * (1 - (order_line.discount or 0.0) / 100.0)
-        taxes = tax_ids.compute_all(price_unit=price, quantity=abs(order_line.qty), currency=self.currency_id, is_refund=order_line.qty<0).get('taxes', [])
+        # The 'is_refund' parameter is used to compute the tax tags. Ultimately, the tags are part
+        # of the key used for summing taxes. Since the POS UI doesn't support the tags, inconsistencies
+        # may arise in 'Round Globally'.
+        if self.company_id.tax_calculation_rounding_method == 'round_globally':
+            is_refund = all(line.qty < 0 for line in order_line.order_id.lines)
+        else:
+            is_refund = order_line.qty < 0
+        taxes = tax_ids.compute_all(price_unit=price, quantity=abs(order_line.qty), currency=self.currency_id, is_refund=is_refund).get('taxes', [])
         date_order = order_line.order_id.date_order
         taxes = [{'date_order': date_order, **tax} for tax in taxes]
         return {