[FIX] purchase: use Product Price precision in bills

When the Product Price precision is greater than the currency precision,
it can lead to incorrect data

To reproduce the error:
(Enable debug mode)
1. Settings > Technical > Database Structure > Decimal Accuracy, edit
Product Price:
    - Digits: 3
2. Create a PO
    - Add a product:
        - Quantity: 12
        - Unit Price: 0.001
3. Save, Confirm, Edit the PO:
    - Qty Received: 12
    - (Note that the total is $0.01)
4. Create a bill:
    - Add the PO to the field "Auto-Complete"

Error: The unit price is $0.000 and so does the total

The rounding of the unit price should be based on the Product Price
precision, not the currency precision.

OPW-2601867

closes odoo/odoo#74627

Signed-off-by: Rémy Voet <ryv-odoo@users.noreply.github.com>