🐛(backend) fix issues with conversion microservice integration

Minor adjustments were needed after working in parallel on two PRs.
The microservice now accepts an API key without requiring it as a Bearer token.

A mistake in reading the microservice response was corrected after refactoring
the serializer to delegate logic to the converter microservice.

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

@@ -25,7 +25,7 @@ def mock_convert_markdown():
     with patch.object(
         YdocConverter,
         "convert_markdown",
-        return_value={"content": "Converted document content"},
+        return_value="Converted document content",
     ) as mock:
         yield mock