[FIX] account, point_of_sale: fix rounding issues with taxes computation (#20573)

Suppose a tax of 20% with price_included = True and apply this tax on 399.99:
- base = 399.99 / 1.2 = 333.32500000000005
- tax_amount = base * 0.2 = 66.665
- round(base) + round(tax_amount) = 333.33 + 66.67 = 400.0 (!= 399.99: WRONG)

-opw: 777925