[FIX] pos_restaurant: validate number of guest put by user

The user could make the mistake to put a large number in the `Guests`
input in the POS order and this is not validated, even an number more large
for the capacity of an integer, raising an uncontrolled error:

`psycopg2.errors.NumericValueOutOfRange: integer out of range`

this commit is adding a validation error in order to limit the number of `Guests`
with the maximum number for an integer:  2**31 - 1

closes odoo/odoo#105393

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