feat(dunning): Allow soft deletion on dunning campaign

app/models/dunning_campaign.rb

@@ -2,6 +2,8 @@
 
 class DunningCampaign < ApplicationRecord
   include PaperTrailTraceable
+  include Discard::Model
+  self.discard_column = :deleted_at
 
   ORDERS = %w[name code].freeze
 
@@ -17,6 +19,7 @@ class DunningCampaign < ApplicationRecord
   validates :max_attempts, numericality: {greater_than: 0}
   validates :code, uniqueness: {scope: :organization_id}, unless: :deleted_at
 
+  default_scope -> { kept }
   scope :applied_to_organization, -> { where(applied_to_organization: true) }
   scope :with_currency_threshold, ->(currencies) {
     joins(:thresholds)

spec/models/dunning_campaign_spec.rb

@@ -17,4 +17,32 @@
   it { is_expected.to validate_numericality_of(:max_attempts).is_greater_than(0) }
 
   it { is_expected.to validate_uniqueness_of(:code).scoped_to(:organization_id) }
+
+  describe "code validation" do
+    let(:code) { "123456" }
+    let(:organization) { create(:organization) }
+
+    it "validates uniqueness of code scoped to organization_id excluding deleted records" do
+      create(:dunning_campaign, code:, organization:)
+      new_record = build(:dunning_campaign, code:, organization:)
+
+      expect(new_record).not_to be_valid
+      expect(new_record.errors[:code]).to include("value_already_exist")
+
+      # Records with deleted_at set should not conflict
+      deleted_record = create(:dunning_campaign, :deleted, code:, organization:)
+      expect(deleted_record).to be_valid
+    end
+  end
+
+  describe "default scope" do
+    let(:deleted_dunning_campaign) { create(:dunning_campaign, :deleted) }
+
+    before { deleted_dunning_campaign }
+
+    it "only returns non-deleted dunning_campaign objects" do
+      expect(described_class.all).to eq([])
+      expect(described_class.with_discarded).to eq([deleted_dunning_campaign])
+    end
+  end
 end