Steps to reproduce:
- Product A with a price-included tax, product B with a negative price and another tax (or none)
- Promotion with a fixed amount discount on specific products (or on the order), both products eligible
- Add both products to a PoS order

Issue:
The discount is split per tax group with a common factor, so the negative line gets a positive reward line, which is expected. But the reward lines add up to less than the discount and the customer pays too much.

Cause:
Since 7280f3597ece each tax group is capped with
`Math.min(get_total_with_tax(), amount)`. A single tax group is compared with the total of the whole order: here the total includes the negative line, so the positive group is lowered while the negative one is kept as is.

Fix:
Apply the cap proportionally: every tax group is scaled by the ratio between the order total and the discountable amount when the total is lower. The scenario of 7280f3597ece gives the same result, and the reward lines add up to the discount whatever the number of tax groups.

opw-6567973

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Forward-Port-Of: odoo/odoo#288985
Forward-Port-Of: odoo/odoo#288853