Don't always remove patients from sessions

If the patient session has been interacted with in some way (either
consent received, or triaged, or even vaccinated) we can't safely remove
the patient from the session as it's unclear what state they're in.

https://good-machine.sentry.io/issues/5990842901

app/models/class_import.rb

@@ -56,10 +56,12 @@ def postprocess_rows!
 
     unknown_patients = session.patients - patients
 
-    unknown_patients.each do |unknown_patient|
-      unknown_patient.update!(school: nil)
-    end
-
-    session.patients.delete(unknown_patients)
+    session
+      .patient_sessions
+      .where(patient: unknown_patients)
+      .find_each do |patient_session|
+        patient_session.patient.update!(school: nil)
+        patient_session.destroy! if patient_session.added_to_session?
+      end
   end
 end

app/models/immunisation_import.rb

@@ -148,9 +148,12 @@ def postprocess_rows!
         .includes(:vaccination_records)
         .select { _1.vaccinated?(programme) }
 
-    PatientSession.where(
-      session: team.sessions.upcoming,
-      patient: already_vaccinated_patients
-    ).delete_all
+    PatientSession
+      .where(
+        session: team.sessions.upcoming,
+        patient: already_vaccinated_patients
+      )
+      .select(&:added_to_session?)
+      .each(&:destroy!)
   end
 end

app/models/session.rb

@@ -131,6 +131,7 @@ def create_patient_sessions!
         required_programmes.all? { |programme| patient.vaccinated?(programme) }
       end
 
+    # First we remove patients from any other upcoming sessions.
     unvaccinated_patients.each do |patient|
       sessions_other_than_self = patient.upcoming_sessions.reject { _1 == self }
       next if sessions_other_than_self.empty?
@@ -142,6 +143,7 @@ def create_patient_sessions!
         .each(&:destroy!)
     end
 
+    # Next we can add the unvaccinated patients to this session.
     PatientSession.import!(
       %i[patient_id session_id],
       unvaccinated_patients.map { [_1.id, id] },

spec/models/class_import_spec.rb

@@ -355,6 +355,20 @@
         expect { record! }.to change { existing_patient.reload.school }.to(nil)
         expect(session.reload.patients).not_to include(existing_patient)
       end
+
+      context "when the patient has been seen" do
+        let(:existing_patient) do
+          create(:patient, :consent_given_triage_not_needed, session:)
+        end
+
+        it "doesn't remove the patient from the session" do
+          expect(session.patients).to include(existing_patient)
+          expect { record! }.to change { existing_patient.reload.school }.to(
+            nil
+          )
+          expect(session.reload.patients).to include(existing_patient)
+        end
+      end
     end
   end
 end