[FIX] purchase_requisition: Compare product lines

Steps to reproduce:
- Create a product with both MTO and Buy routes.
- Create a Sale Order containing this product.
- On the created PO, go to Alternatives -> Create Alternative and select
another vendor.
- Go back to the original Sale Order
- Click on the linked Purchase Order
- Go to Alternatives -> Compare Product Lines

When doing this, the `active_id` in the context is the id of the Sale
Order, which raises an issue in the renderer for this list as it's using
the active_id as if it was the Purchase Order.

closes odoo/odoo#106085

Signed-off-by: Tiffany Chang <tic@odoo.com>