[IMP] mail: call _add_default_followers only once in create
[IMP] mail: call _add_default_followers only once in create

There was no reason to call the method many times in the loop as its result
does not depend on `vals_list`.

PR: #36654
Part of task-2070602
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo