Fix ArgumentError when loading newer Private Mentions (#25399)

spec/controllers/api/v1/conversations_controller_spec.rb

@@ -52,5 +52,23 @@ RSpec.describe Api::V1::ConversationsController do
         end
       end
     end
+
+    context 'with since_id' do
+      context 'when requesting old posts' do
+        it 'returns conversations' do
+          get :index, params: { since_id: Mastodon::Snowflake.id_at(1.hour.ago, with_random: false) }
+          json = body_as_json
+          expect(json.size).to eq 1
+        end
+      end
+
+      context 'when requesting posts in the future' do
+        it 'returns no conversation' do
+          get :index, params: { since_id: Mastodon::Snowflake.id_at(1.hour.from_now, with_random: false) }
+          json = body_as_json
+          expect(json.size).to eq 0
+        end
+      end
+    end
   end
 end