[FIX] purchase_stock: small discount at invoicing

- Create stockable product P: FIFO, Automated Valuation
- Set the price difference account
- Configure Decimal Accuracy for Discounts to 5.
- Create a PO
- Add a line with:
  Product P
  Quantity: 10,000
  Price unit: 100
  A tax
- Confirm the PO, receive the product and create the invoice
- Set a discount of 0.92431 on the invoice line
- Post the invoice

The journal entries for the invoices contains the price difference move,
the amount must be 9,243.10 but is 9,200.00.

In case taxes are defined on the the invoice line, we call `compute_all`
which will round the `price_unit`. In this case, we don't want to round
it.

Fixes #56832
opw-2329028

X-original-commit: 498238fb02c32285f9d9ac779884105ae9eaa1e4