[FIX] purchase_stock: consider product unit price precision

When changing the product price precision, this can lead to incorrect
stock valuations.

To reproduce the error:
(Enable debug mode)
1. Go to Settings > Technical > Database Strucutre > Decimal Accuracy
2. Edit Product Price:
    - Digits: 4
3. Create a Product Category PC:
    - Costing Method: FIFO
4. Create a Product P:
    - Product Type: Storable Product
    - Product Category: PC
5. Create a RfQ with product P:
    - Quantity: 1000
    - Unit Price: 0.035
6. Confirm Order, Receive Products, Validate
7. Click on Valuation

Error: The total value is equal to $40 instead of $35. The calculation
was done after rounding the unit price: $0.035 becomes $0.04, then
1000*0.04=$40.

When confirming the RfQ, a stock move is created. To do so, the method
`_get_stock_move_price_unit` is called. When validating the delivery, it
recomputes the unit price thanks to method `_get_price_unit`. In both
situation, and if the line has taxes, the method `compute_all` is called
like this:
```python
price_unit = line.taxes_id.with_context(round=False).compute_all(price_unit,
	currency=line.order_id.currency_id, quantity=1.0)['total_void']
```
But here is the problem. In this method, total amount is computed with
this line:
```python
base = currency.round(price_unit * quantity)
```
However, `quantity` is equal to 1 and the multiplication is rounded
using the currency precision. As a result, `base` is equal to $0.04.
Then, all computations will use this value and will be incorrect.

This fix applies the real quantity so `base` will have the correct
value:
```
base = currency.round(price_unit * quantity)
     = currency.round(0.035 * 1000)
     = 35
```

OPW-2472192

closes odoo/odoo#69297

Signed-off-by: William Henrotin <Whenrow@users.noreply.github.com>