chore: Add assigned_count to conversation APIs (#2665)

spec/finders/conversation_finder_spec.rb

@@ -55,6 +55,16 @@ describe ::ConversationFinder do
         result = conversation_finder.perform
         expect(result[:conversations].count).to be 3
       end
+
+      it 'returns the correct meta' do
+        result = conversation_finder.perform
+        expect(result[:count]).to eq({
+                                       mine_count: 2,
+                                       assigned_count: 3,
+                                       unassigned_count: 1,
+                                       all_count: 4
+                                     })
+      end
     end
 
     context 'with team' do