Adds `id` in the `mail.activity`'s order to force deterministic order in
case multiple activities have the same deadline.
Not having a deterministic order can cause issue when tests check
activities' values but don't ensure they are sorted also by id.
closesodoo/odoo#120034
X-original-commit: b0a99bde8efef287f3ed89afb9da2a67233d4bd8
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>