🚧(backend) student_signed_on with signature provider on serializer

For the serializer client ContractLightSerializer, whether the
notification from the signature provider takes some time
to let us know that the student has signed, when the viewset is called,
it will trigger our serializer method that will check if the
student has finished signing his part on the document.
That allows the frontend to get the latest information when
the student gets to his dashboard of course orders.

src/backend/joanie/core/serializers/client.py

@@ -17,6 +17,7 @@
 from joanie.core.serializers.base import CachedModelSerializer
 from joanie.core.serializers.fields import ISO8601DurationField, ThumbnailDetailField
 from joanie.payment.models import CreditCard
+from joanie.signature.backends import get_signature_backend
 
 
 class AbilitiesModelSerializer(serializers.ModelSerializer):
@@ -446,11 +447,30 @@ class Meta:
 class ContractLightSerializer(serializers.ModelSerializer):
     """Light serializer for Contract model."""
 
+    student_signed_on = serializers.SerializerMethodField()
+
     class Meta:
         model = models.Contract
         fields = ["id", "organization_signed_on", "student_signed_on"]
         read_only_fields = fields
 
+    def get_student_signed_on(self, contract):
+        """
+        Returns if the student has signed the document.
+        """
+        if (
+            contract.submitted_for_signature_on
+            and not contract.student_signed_on
+            and not contract.organization_signed_on
+        ):
+            signature_backend = get_signature_backend()
+            signature_state = signature_backend.get_signature_state(
+                reference_id=contract.signature_backend_reference
+            )
+            return signature_state.get("student")
+
+        return contract.student_signed_on
+
 
 class ContractSerializer(AbilitiesModelSerializer):
     """Serializer for Contract model serializer"""