chore: Add validations to campaign model

- Add validations to campaign model ensuring that the associated inbox belongs to one with in the campaign account.

app/models/campaign.rb

@@ -38,6 +38,7 @@ class Campaign < ApplicationRecord
   validate :validate_url
   validate :prevent_completed_campaign_from_update, on: :update
   validate :sender_must_belong_to_account
+  validate :inbox_must_belong_to_account
 
   belongs_to :account
   belongs_to :inbox
@@ -92,6 +93,14 @@ class Campaign < ApplicationRecord
     errors.add(:url, 'invalid') if inbox.inbox_type == 'Website' && !use_http_protocol
   end
 
+  def inbox_must_belong_to_account
+    return unless inbox
+
+    return if inbox.account_id == account_id
+
+    errors.add(:inbox_id, 'must belong to the same account as the campaign')
+  end
+
   def sender_must_belong_to_account
     return unless sender