[FIX] point_of_sale: prevent saving a finalized order to unpaid orders

Before this commit: it was possible to use the pos reference of a validated
order for another order. It causes the order to miss because the server
wouldn't save a new order if there is an existing order with the same pos
reference.

Steps to reproduce the issue on the runbot:

	Open a PoS session
	Throttling the network
	Create an order by selecting some items
	Pay the order
	Validate the order and just before it finishes the job, go back
	Now it's not possible to add an item or change the quantity
	Now click on one of the order lines

	The order will save to unpaid order, and if you reload the page, it will
be possible to modify the order and use it for a new PoS order.

Solution

	The solution is to prevent it from saving it again.

opw-2951938

closes odoo/odoo#99472

Signed-off-by: Masereel Pierre <pim@odoo.com>