fix: group offer terminated scope and terminated? method

- scope was querying for non existent field
- method was validating nonexistent attribute

app/models/group_offer.rb

@@ -70,6 +70,8 @@ class GroupOffer < ApplicationRecord
     date_between_inclusion(:period_end, start_date, end_date)
   }
 
+  scope :terminated, (-> { field_not_nil(:period_end_set_by) })
+
   def active_group_assignments_between?(start_date, end_date)
     group_assignments.active_between(start_date, end_date).any?
   end
@@ -78,6 +80,10 @@ class GroupOffer < ApplicationRecord
     group_assignments.have_start.any? || group_assignment_logs.any?
   end
 
+  def terminated?
+    period_end_set_by.present?
+  end
+
   def all_group_assignments_ended_within?(start_date, end_date)
     ended_within = group_assignments.end_within(start_date, end_date).ids
     not_end_before = group_assignments.end_after(end_date).ids