fix: Adds assignee as a participant during auto-assign (#6789)

- Ensures that the assignee is added as a participant during auto-assign

app/models/concerns/auto_assignment_handler.rb

@@ -15,6 +15,7 @@ module AutoAssignmentHandler
     return unless should_run_auto_assignment?
 
     ::AutoAssignment::AgentAssignmentService.new(conversation: self, allowed_agent_ids: inbox.member_ids_with_assignment_capacity).perform
+    conversation_participants.find_or_create_by(user_id: assignee_id) if assignee_id.present?
   end
 
   def should_run_auto_assignment?

app/services/messages/mention_service.rb

@@ -42,7 +42,7 @@ class Messages::MentionService
 
   def add_mentioned_users_as_participants(validated_mentioned_ids)
     validated_mentioned_ids.each do |user_id|
-      message.conversation.conversation_participants.find_or_create_by!(user_id: user_id)
+      message.conversation.conversation_participants.find_or_create_by(user_id: user_id)
     end
   end
 end

spec/models/concerns/auto_assignment_handler_shared.rb

@@ -26,6 +26,10 @@ shared_examples_for 'auto_assignment_handler' do
       expect(conversation.reload.assignee).to eq(agent)
     end
 
+    it 'adds assignee to conversation participants' do
+      expect(conversation.conversation_participants.map(&:user)).to include(agent)
+    end
+
     it 'will not auto assign agent if enable_auto_assignment is false' do
       inbox.update(enable_auto_assignment: false)