♻️(backend) return the full document in the duplicate response

The duplicate endpoint used to respond with only `{"id": ...}`. It now
returns the complete duplicated document representation, consistent
with the other document detail endpoints, so the frontend doesn't have
to make a follow-up request to get the new document's data.

This required setting `is_favorite` explicitly on the duplicated
document before serializing it: it is normally set by the
`annotate_is_favorite` queryset method, which the newly created
document never goes through. Being a read-only serializer field, it
was silently dropped from the response instead of raising an error. A
document can't be a favorite right after being created, so it is set
to `False` directly.

src/backend/core/api/viewsets.py

@@ -1349,10 +1349,16 @@ class DocumentViewSet(
             document=duplicated_document,
         )
 
-        return drf_response.Response(
-            {"id": str(duplicated_document.id)}, status=status.HTTP_201_CREATED
+        # Set the `is_favorite` attribute to False for the duplicated document, as it
+        # cannot be a favorite immediately after creation.
+        duplicated_document.is_favorite = False
+
+        serializer = serializers.DocumentSerializer(
+            duplicated_document, context=self.get_serializer_context()
         )
 
+        return drf_response.Response(serializer.data, status=status.HTTP_201_CREATED)
+
     def _duplicate_document(
         self,
         document_to_duplicate,

src/backend/core/tests/documents/test_api_documents_duplicate.py

@@ -108,6 +108,8 @@ def test_api_documents_duplicate_success(index):
         users=[user, factories.UserFactory()],
         title="document with an image",
         attachments=[key for key, _ in image_refs],
+        # The original document is a favorite: the duplicate should not be one
+        favorited_by=[user],
     )
     factories.DocumentFactory(id=document_ids[(index + 1) % 3])
     # Don't create document for third ID to check that it doesn't impact access to attachments
@@ -131,6 +133,30 @@ def test_api_documents_duplicate_success(index):
     assert duplicated_document.get_parent() == document.get_parent()
     assert duplicated_document.path == document.get_last_sibling().path
 
+    assert response.json() == {
+        "id": str(duplicated_document.id),
+        "abilities": duplicated_document.get_abilities(user),
+        "ancestors_link_reach": None,
+        "ancestors_link_role": None,
+        "computed_link_reach": duplicated_document.computed_link_reach,
+        "computed_link_role": duplicated_document.computed_link_role,
+        "created_at": duplicated_document.created_at.isoformat().replace("+00:00", "Z"),
+        "creator": str(user.id),
+        "deleted_at": None,
+        "depth": duplicated_document.depth,
+        "excerpt": duplicated_document.excerpt,
+        "is_favorite": False,
+        "link_reach": duplicated_document.link_reach,
+        "link_role": duplicated_document.link_role,
+        "nb_accesses_ancestors": duplicated_document.nb_accesses_ancestors,
+        "nb_accesses_direct": duplicated_document.nb_accesses_direct,
+        "numchild": 0,
+        "path": duplicated_document.path,
+        "title": "Copy of document with an image",
+        "updated_at": duplicated_document.updated_at.isoformat().replace("+00:00", "Z"),
+        "user_role": "owner",
+    }
+
     mock_capture.assert_called_once_with(
         "doc_duplicated",
         user,