Make admin contacts optional for private registrants

This change makes admin contacts optional for private registrants while keeping them mandatory for organizations. The changes include:

- Updated Domain model validations to make admin and tech contacts optional (min=0) for private registrants
- Added validation rules methods to handle different requirements based on registrant type
- Modified EPP domain creation to support domains without admin contacts for private registrants
- Updated attach_default_contacts to skip adding contacts for private registrants
- Added comprehensive test coverage for:
  - Domain model validations with private/org registrants
  - EPP domain creation without admin contacts for private registrants
  - REPP API contact management for private registrants

This implements the requirement to make admin contacts optional for private registrations of .ee domains while maintaining the existing validation rules for organizations.

test/integration/repp/v1/domains/contacts_test.rb

@@ -108,9 +108,32 @@ class ReppV1DomainsContactsTest < ActionDispatch::IntegrationTest
     refute @domain.tech_contacts.find_by(code: contact.code).present?
   end
 
+  def test_can_remove_all_admin_contacts_for_private_registrant
+    Spy.on_instance_method(Actions::DomainUpdate, :validate_email).and_return(true)
+
+    @domain.registrant.update!(ident_type: 'priv')
+    @domain.reload
+    assert_not @domain.registrant.org?
+
+    contact = @domain.admin_contacts.last
+
+    payload = { contacts: [ { code: contact.code, type: 'admin' } ] }
+    delete "/repp/v1/domains/#{@domain.name}/contacts", headers: @auth_headers, params: payload
+    json = JSON.parse(response.body, symbolize_names: true)
+
+    @domain.reload
+    assert_response :ok
+    assert_equal 1000, json[:code]
+
+    assert_empty @domain.admin_contacts
+  end
+
   def test_can_not_remove_one_and_only_contact
     Spy.on_instance_method(Actions::DomainUpdate, :validate_email).and_return(true)
 
+    @domain.registrant.update!(ident_type: 'org')
+    @domain.reload
+
     contact = @domain.admin_contacts.last
 
     payload = { contacts: [ { code: contact.code, type: 'admin' } ] }