Assert the waiting-list promotion email on reject

spec/controllers/workshop_invitation_controller_spec.rb

@@ -8,6 +8,13 @@
 
   before { login(member) }
 
+  # The invitation mailer sends multipart/mixed > multipart/alternative > text/html;
+  # the root body is empty, so read the nested html part.
+  def html_body(mail)
+    parts = mail.parts.flat_map { |part| part.multipart? ? part.parts : [part] }
+    parts.find { |part| part.content_type.match?('text/html') }&.body&.decoded
+  end
+
   describe 'GET #show' do
     it 'returns http success' do
       get :show, params: { id: invitation.token }
@@ -181,6 +188,27 @@
         post :reject, params: { id: invitation.token }
         expect(waitlisted_invitation.reload.attending).to be true
       end
+
+      it 'emails the promoted member a confirmation they are attending' do
+        post :reject, params: { id: invitation.token }
+
+        mail = ActionMailer::Base.deliveries.find { |m| m.to.include?(waitlisted_member.email) }
+        expect(mail).not_to be_nil
+        expect(html_body(mail)).to include('been confirmed')
+      end
+
+      it 'sends the promotion variant of the email (waiting-list flag set)' do
+        post :reject, params: { id: invitation.token }
+
+        mail = ActionMailer::Base.deliveries.find { |m| m.to.include?(waitlisted_member.email) }
+        expect(html_body(mail)).to include('A spot became available and your attendance has now been confirmed!')
+      end
+
+      it 'does not email anyone else (the rejecting member gets no promotion copy)' do
+        expect do
+          post :reject, params: { id: invitation.token }
+        end.to change { ActionMailer::Base.deliveries.count }.by(1)
+      end
     end
 
     context 'when a coach is waitlisted and a student seat frees up' do