[FIX] website_sale: return ID instead of record during create
[FIX] website_sale: return ID instead of record during create

partner_id was either a recordset (in create mode), either an id (in edit more)
in the `_checkout_form_save` method.

An id is expected as the result is used in the ORM method (4, partner_id)

Closes #18533
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo