Standardize uniqueness validation declaration on Mention (#33247)

spec/models/mention_spec.rb

@@ -3,8 +3,14 @@
 require 'rails_helper'
 
 RSpec.describe Mention do
-  describe 'validations' do
+  describe 'Associations' do
     it { is_expected.to belong_to(:account).required }
     it { is_expected.to belong_to(:status).required }
   end
+
+  describe 'Validations' do
+    subject { Fabricate.build :mention }
+
+    it { is_expected.to validate_uniqueness_of(:account_id).scoped_to(:status_id) }
+  end
 end