[FIX] account: Fix price_unit rounding issue with fpos/price included tax

- Create an invoice with an empty fiscal position
- Create a line with a product having 100.0 as sale price and 21.0% price-included tax
=> price_unit equals 99.99

This is because 100 / 1.21 ~= 82.64 but 82.64 * 1.21 ~= 99.99 != 100.0.
The bug only appears when managing a fiscal position because the code is trying to adapt the product price_unit to the newly computed taxes.

closes odoo/odoo#69522

Signed-off-by: oco-odoo <oco-odoo@users.noreply.github.com>