♻️(backend) refactor confirm_student_signature in base signature backend

We needed to update the was we retrieve the batch order from a contract
into the signature backend.

Author: jonathanreveille

src/backend/joanie/signature/backends/base.py

@@ -51,8 +51,7 @@ def _confirm_student_signature(self, contract):
         contract.student_signed_on = django_timezone.now()
         contract.save()
 
-        if contract.batch_orders.count() == 1:
-            batch_order = contract.batch_orders.first()
+        if batch_order := contract.batch_order:
             flow = batch_order.flow
             if batch_order.uses_purchase_order:
                 flow.update()  # Transition to `signing` state now and second call is to `completed`

src/backend/joanie/signature/backends/dummy.py

@@ -62,7 +62,7 @@ def get_signature_invitation_link(self, recipient_email: str, reference_ids: lis
             "finished" if contract.student_signed_on is not None else "signed"
         )
         # Case for batch order's contract
-        if contract.batch_orders.exists() and event_target == "signed":
+        if contract.batch_order and event_target == "signed":
             self.confirm_signature(reference_id)
 
         return (
@@ -80,7 +80,7 @@ def delete_signing_procedure(self, reference_id: str):
         # For case of batch order, we should take away the signature of the buyer because
         # it's marked when calling `get_signature_invitation_link`
         contract = models.Contract.objects.get(signature_backend_reference=reference_id)
-        if contract.batch_orders.exists():
+        if contract.batch_order:
             contract.student_signed_on = None
             contract.save()