chore(providers): Return 409 on conflict (#10293)

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

api/src/backend/api/tests/test_views.py

@@ -1342,7 +1342,11 @@ class TestProviderViewSet:
         response = authenticated_client.post(
             reverse("provider-list"), data=provider_json_payload, format="json"
         )
-        assert response.status_code == status.HTTP_400_BAD_REQUEST
+        assert response.status_code == status.HTTP_409_CONFLICT
+        error = response.json()["errors"][0]
+        assert error["detail"] == "Provider already exists."
+        assert error["code"] == "conflict"
+        assert error["source"]["pointer"] == "/data/attributes/uid"
 
         mock_delete_task.reset_mock()
         mock_delete_task.return_value = task_mock

api/src/backend/api/v1/serializers.py

@@ -6,6 +6,7 @@ from django.conf import settings
 from django.contrib.auth import authenticate
 from django.contrib.auth.models import update_last_login
 from django.contrib.auth.password_validation import validate_password
+from django.core.exceptions import ValidationError as DjangoValidationError
 from django.db import IntegrityError
 from drf_spectacular.utils import extend_schema_field
 from jwt.exceptions import InvalidKeyError
@@ -959,6 +960,26 @@ class ProviderCreateSerializer(RLSSerializer, BaseWriteSerializer):
             },
         }
 
+    def create(self, validated_data):
+        try:
+            return super().create(validated_data)
+        except DjangoValidationError as e:
+            if "unique_provider_uids" in str(e):
+                raise ConflictException(
+                    detail="Provider already exists.",
+                    pointer="/data/attributes/uid",
+                )
+            raise
+        except IntegrityError as e:
+            # Handle race conditions where the unique constraint is enforced at the DB level
+            # after validation has already passed.
+            if "unique_provider_uids" in str(e):
+                raise ConflictException(
+                    detail="Provider already exists.",
+                    pointer="/data/attributes/uid",
+                )
+            raise
+
 
 class ProviderUpdateSerializer(BaseWriteSerializer):
     """