[FIX] sale{,_stock}: decrease qty of SOL

When decreasing the qty of a SOL, if there is already a packaging
defined, a return will be created for the difference

To reproduce the issue:
1. Create a product P
    - Storable
    - With one packaging PK for 10 x P
2. Create and confirm a SO
    - Order Lines:
        - 10 x P with PK
3. On the SO, decrease the quantity of P to 8

Error: a return is created for the difference. Instead, the existing
picking should be updated

At some point, we try to merge the stock moves (`10 x P` and `-2 x P`)
and, to do so, they must have the same `product_packaging_id`:
https://github.com/odoo/odoo/blob/08f53ffa30479cfb452ebaed61b42655eb0e6a95/addons/stock/models/stock_move.py#L808-L813

However, on the SO, when decreasing the quantity, an onchange tries to
find the best packaging. Because the new quantity is not a multiple of
PK, this one is not returned. As a result, the SO has no more packaging
and so does the new stock move (`-2 x P`). Therefore, the stock moves
can not be merged and a new picking (the return) is created.

This commit updates the onchange method so it behaves the same way as
purchase side:
https://github.com/odoo/odoo/blob/20fa7c6f0f5c9cdfae2519ea8d0ff849d7fa3b9e/addons/purchase/models/purchase.py#L1224-L1231
In this onchange, the same diff has recently be added for another reason
(see [1])

[1] 20fa7c6f0f5c9cdfae2519ea8d0ff849d7fa3b9e

OPW-3027110

X-original-commit: e92e960a36babf4daf8f79d210c7f53fb376aa74
Part-of: odoo/odoo#107045

addons/sale/models/sale_order_line.py

@@ -617,7 +617,7 @@ class SaleOrderLine(models.Model):
             # Find biggest suitable packaging
             if line.product_id and line.product_uom_qty and line.product_uom:
                 line.product_packaging_id = line.product_id.packaging_ids.filtered(
-                    'sales')._find_suitable_product_packaging(line.product_uom_qty, line.product_uom)
+                    'sales')._find_suitable_product_packaging(line.product_uom_qty, line.product_uom) or line.product_packaging_id
 
     @api.depends('product_packaging_id', 'product_uom', 'product_uom_qty')
     def _compute_product_packaging_qty(self):

addons/sale_stock/tests/test_sale_stock.py

@@ -1292,3 +1292,26 @@ class TestSaleStock(TestSaleCommon, ValuationReconciliationTestCommon):
         so.order_line.product_uom_qty = 8
         self.assertRecordValues(so.picking_ids, [{'location_id': warehouse.lot_stock_id.id, 'location_dest_id': customer_location.id}])
         self.assertEqual(so.picking_ids.move_ids.product_uom_qty, 8)
+
+    def test_packaging_and_qty_decrease(self):
+        packaging = self.env['product.packaging'].create({
+            'name': "Super Packaging",
+            'product_id': self.product_a.id,
+            'qty': 10.0,
+        })
+
+        so_form = Form(self.env['sale.order'])
+        so_form.partner_id = self.partner_a
+        with so_form.order_line.new() as line:
+            line.product_id = self.product_a
+            line.product_uom_qty = 10
+        so = so_form.save()
+        so.action_confirm()
+
+        self.assertEqual(so.order_line.product_packaging_id, packaging)
+
+        with Form(so) as so_form:
+            with so_form.order_line.edit(0) as line:
+                line.product_uom_qty = 8
+
+        self.assertEqual(so.picking_ids.move_ids.product_uom_qty, 8)