[FIX] point_of_sale: handle line breaks in product search

Before this commit: the `product.display_name` field could contain
line breaks, which prevented it from being matched correctly by
regular expressions. This commit fixes the issue by modifying the
`_product_search_string()` function to remove line breaks.

opw-3239830

closes odoo/odoo#116692

Signed-off-by: Trinh Jacky (trj) <trj@odoo.com>