fix: downcase email before finding (#8921)

* fix: downcase email when finding

* feat: add `from_email` class

* refactor: use `from_email`

* feat: add rule to disallow find_by email directly

* chore:  remove redundant test

Since the previous imlpmentation didn't do a case-insentive search, a new user would be created, and the error would be raised at the DB layer. With the new changes, this test case is redundant

* refactor: use from_email

app/controllers/devise_overrides/sessions_controller.rb

@@ -33,7 +33,7 @@ class DeviseOverrides::SessionsController < DeviseTokenAuth::SessionsController
   def process_sso_auth_token
     return if params[:email].blank?
 
-    user = User.find_by(email: params[:email])
+    user = User.from_email(params[:email])
     @resource = user if user&.valid_sso_auth_token?(params[:sso_auth_token])
   end
 end