feat: Add a sort option for conversations waiting for a reply from an agent (#7364)

spec/models/conversation_spec.rb

@@ -32,6 +32,10 @@ RSpec.describe Conversation do
       expect(conversation.display_id).to eq(1)
     end
 
+    it 'sets waiting since' do
+      expect(conversation.waiting_since).not_to be_nil
+    end
+
     it 'creates a UUID for every conversation automatically' do
       uuid_pattern = /[0-9a-f]{8}\b-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-\b[0-9a-f]{12}$/i
       expect(conversation.uuid).to match(uuid_pattern)
@@ -523,6 +527,7 @@ RSpec.describe Conversation do
         contact_last_seen_at: conversation.contact_last_seen_at.to_i,
         agent_last_seen_at: conversation.agent_last_seen_at.to_i,
         created_at: conversation.created_at.to_i,
+        waiting_since: conversation.waiting_since.to_i,
         priority: nil,
         unread_count: 0
       }