chore: revert memoization patterns and redundant default values

- Disable Style/MemoizeInstanceVariable and Style/RedundantAssignment rules
- Revert all instance_variable_defined? patterns back to ||= memoization
- Restore || 0 and || 1 default values that were removed by RuboCop
- Restore ternary operator formatting
- Reduce PR from 35 to 18 files by removing cosmetic style changes

app/controllers/api/v1/accounts/campaigns_controller.rb

@@ -24,11 +24,7 @@ class Api::V1::Accounts::CampaignsController < Api::V1::Accounts::BaseController
   private
 
   def campaign
-    if instance_variable_defined?(:@campaign)
-      @campaign
-    else
-      @campaign = Current.account.campaigns.find_by(display_id: params[:id])
-    end
+    @campaign ||= Current.account.campaigns.find_by(display_id: params[:id])
   end
 
   def campaign_params

app/controllers/api/v1/accounts/categories_controller.rb

@@ -39,11 +39,7 @@ class Api::V1::Accounts::CategoriesController < Api::V1::Accounts::BaseControlle
   end
 
   def portal
-    if instance_variable_defined?(:@portal)
-      @portal
-    else
-      @portal = Current.account.portals.find_by(slug: params[:portal_id])
-    end
+    @portal ||= Current.account.portals.find_by(slug: params[:portal_id])
   end
 
   def related_categories_records

app/controllers/api/v1/accounts/conversations/direct_uploads_controller.rb

@@ -13,10 +13,6 @@ class Api::V1::Accounts::Conversations::DirectUploadsController < ActiveStorage:
   private
 
   def conversation
-    if instance_variable_defined?(:@conversation)
-      @conversation
-    else
-      @conversation = Current.account.conversations.find_by(display_id: params[:conversation_id])
-    end
+    @conversation ||= Current.account.conversations.find_by(display_id: params[:conversation_id])
   end
 end

app/controllers/api/v1/accounts/integrations/shopify_controller.rb

@@ -46,11 +46,7 @@ class Api::V1::Accounts::Integrations::ShopifyController < Api::V1::Accounts::Ba
   end
 
   def contact
-    if instance_variable_defined?(:@contact)
-      @contact
-    else
-      @contact = Current.account.contacts.find_by(id: params[:contact_id])
-    end
+    @contact ||= Current.account.contacts.find_by(id: params[:contact_id])
   end
 
   def fetch_hook

app/controllers/api/v1/widget/base_controller.rb

@@ -25,11 +25,7 @@ class Api::V1::Widget::BaseController < ApplicationController
   end
 
   def inbox
-    if instance_variable_defined?(:@inbox)
-      @inbox
-    else
-      @inbox = ::Inbox.find_by(id: auth_token_params[:inbox_id])
-    end
+    @inbox ||= ::Inbox.find_by(id: auth_token_params[:inbox_id])
   end
 
   def conversation_params