Changing the notification preference silently promoted a light user to a regular user, granting them access rights they were never meant to have.

Steps to reproduce:
- log in as a light user (light/light)
- open My Profile
- in Preferences, switch "Notification" from "Email" to "Odoo"
- log back in as an administrator and open that user's form
- the Role field now reads "User" instead of "Light"

The notification preference is not a plain field: choosing "Odoo" puts the user in a dedicated technical group. That group grants no access right of its own, but a user is considered light only as long as every group they hold is explicitly known to be a light one. Since this group was never declared as such, it was treated like any ordinary access group, which was enough to have the user reclassified as regular.

This commit declares the group as a light one, so the preference stays a preference and no longer affects the user's role. A regression test covers both directions of the toggle for a light and a regular user, checking the role, the underlying group and the role search filter.

Note that existing databases need a `mail` upgrade to clear the stale link.

Task-6575413

Forward-Port-Of: odoo/odoo#288646