Add validation to additional pack purchase_xid

For GitLab.com we should attempt to ensure the uniqueness
of Ci::Minutes::AdditionalPack purchase_xid values

Changelog: changed
EE: true

ee/app/models/ci/minutes/additional_pack.rb

@@ -15,6 +15,7 @@ module Ci
       validates :namespace, :number_of_minutes, presence: true
       validates :expires_at, :purchase_xid, presence: true, if: -> { ::Gitlab.com? }
       validates :purchase_xid, length: { maximum: 32 }
+      validates :purchase_xid, uniqueness: true, if: -> { ::Gitlab.com? }
     end
   end
 end

ee/spec/models/ci/minutes/additional_pack_spec.rb

@@ -8,6 +8,8 @@ RSpec.describe Ci::Minutes::AdditionalPack do
   end
 
   describe 'validations' do
+    subject(:additional_pack) { build(:ci_minutes_additional_pack) }
+
     it { is_expected.to validate_presence_of(:namespace) }
     it { is_expected.to validate_presence_of(:number_of_minutes) }
     it { is_expected.to validate_length_of(:purchase_xid).is_at_most(32) }
@@ -19,6 +21,7 @@ RSpec.describe Ci::Minutes::AdditionalPack do
 
       it { is_expected.to validate_presence_of(:expires_at) }
       it { is_expected.to validate_presence_of(:purchase_xid) }
+      it { is_expected.to validate_uniqueness_of(:purchase_xid) }
     end
 
     context 'when self-managed' do
@@ -28,6 +31,7 @@ RSpec.describe Ci::Minutes::AdditionalPack do
 
       it { is_expected.not_to validate_presence_of(:purchase_xid) }
       it { is_expected.not_to validate_presence_of(:expires_at) }
+      it { is_expected.not_to validate_uniqueness_of(:purchase_xid) }
     end
   end
 end