[FIX] product: variant num w/o prefetch pollution

When getting:

- product_variant_count,
- sales_count,

of a product.product, we would pollute the records to be prefetched by
all the variants when counting the number of variants.

Thus if this happened before sales_count, we would possibly compute the
sales_count for up to 1000 records when it could have been needed for
just one.

By using `.with_prefetch()`, a recordset will have its own records to be
prefetched list and will not pollute the original one.

opw-1865111
fixes #25649
closes #23112 (PR with similar fix)
closes #25741

addons/product/models/product_template.py

@@ -250,7 +250,8 @@ class ProductTemplate(models.Model):
     @api.one
     @api.depends('product_variant_ids.product_tmpl_id')
     def _compute_product_variant_count(self):
-        self.product_variant_count = len(self.product_variant_ids)
+        # do not pollute variants to be prefetched when counting variants
+        self.product_variant_count = len(self.with_prefetch().product_variant_ids)
 
     @api.depends('product_variant_ids', 'product_variant_ids.default_code')
     def _compute_default_code(self):