[FIX] l10n_it_edi: we need for precision in the price unit in case e.g. of tax included

According to the spec, the PrezzoUnitario should be the unit price
without the taxes.  If we use taxes included, this can be problematic.
That is because there is another rule that says that the PrezzoUnitario
* Quantita - Sconto should be within 0.01 precision of the PrezzoTotale.

So, with taxes included and a price unit only rounded to 2 decimals, you
easily go out of that limit.

The solution is to increase the precision on the price_unit (6 decimals
should not cause side effects and precise enough for most cases)
and to do the calculation differently by calculating it again from the
price_subtotal on the invoice line.

Task id: 2764978

closes odoo/odoo#85718

X-original-commit: 82f2794a2aee8492cbd36d8ab4d8a62c9665789d
Signed-off-by: Laurent Smet <las@odoo.com>
Signed-off-by: Josse Colpaert <jco@odoo.com>

addons/l10n_it_edi/data/invoice_it_template.xml

@@ -20,7 +20,7 @@
                 </Descrizione>
                 <Quantita t-esc="format_numbers(line.quantity)"/>
                 <UnitaMisura t-if="line.product_uom_id.category_id != env.ref('uom.product_uom_categ_unit')" t-esc="line.product_uom_id.name"/>
-                <PrezzoUnitario t-esc="format_monetary(taxes['total_excluded'], currency)"/>
+                <PrezzoUnitario t-esc="'%.06f' % (line.price_subtotal / (( 1 - (line.discount or 0.0) / 100.0) * line.quantity))"/>
                 <ScontoMaggiorazione t-if="line.discount != 0">
                     <!-- [2.2.1.10] -->
                     <Tipo t-esc="discount_type(line.discount)"/>