Fix flaky manage attendance feature test (#2774)

Instead of picking the last patient (which doesn't have a guaranteed
order) we can pick the patient that will be left on the page, i.e. the
patient that's missing attendance information.

This has been extracted from #2771.

Example failures:

-
https://github.com/nhsuk/manage-vaccinations-in-schools/actions/runs/12389409160/attempts/2
-
https://github.com/nhsuk/manage-vaccinations-in-schools/actions/runs/12389403286/attempts/2

spec/features/manage_attendance_spec.rb

@@ -94,7 +94,12 @@ def when_i_register_a_patient_as_absent
   end
 
   def when_i_go_to_a_patient
-    click_link @session.reload.patients.last.full_name
+    click_link PatientSession
+                 .where
+                 .missing(:session_attendances)
+                 .find_by(session: @session)
+                 .patient
+                 .full_name
   end
 
   def then_the_patient_is_not_registered_yet