Remove default value for mou_signature agreement_type

We set a default value for the agreement_type when we added the column
so we pre-populated existing records with that value. Remove the
default value to make sure we're always deliberately setting the
correct agreement_type for new records.

Author: stephencdaly

db/migrate/20260316122813_remove_default_value_for_mou_signature_agreement_type.rb

@@ -0,0 +1,9 @@
+class RemoveDefaultValueForMouSignatureAgreementType < ActiveRecord::Migration[8.1]
+  def up
+    change_column_default :mou_signatures, :agreement_type, nil
+  end
+
+  def down
+    change_column_default :mou_signatures, :agreement_type, "crown"
+  end
+end

db/schema.rb

@@ -28,7 +28,7 @@
                                 research_contact_status: :consented,
                                 user_research_opted_in_at: Time.zone.now })
 
-  MouSignature.create! user: default_user, organisation: gds
+  MouSignature.create! user: default_user, organisation: gds, agreement_type: "crown"
 
   # create extra organisations
   test_org = Organisation.create! slug: "test-org", name: "Test Org", abbreviation: "TO"