Detect infinite recursion in Entity.to_xml

Summary

Addresses https://gitlab.indiscale.com/caosdb/customers/f-fit/management/-/issues/96 / https://gitlab.com/linkahead/linkahead-pylib/-/issues/73
When calling to_xml on an Entity that refers to itself, to_xml now detects the infinite recursion and stops after the first level of repeated information.

Focus

• What is the likelihood that a version refers to itself (predecessor / successor)? Do we need to add the same functionality to Version.to_xml?
• Does this change anything for Entities that could already successfully call to_xml before? Are there cases where this might break the communication with the server?

Test Environment

Manual Testing

Check List for the Author

• All automated tests pass
• Reference related issues
• Up-to-date CHANGELOG.md (or not necessary)
• Up-to-date JSON schema (or not necessary)
• Appropriate user and developer documentation (or not necessary)
• Annotations in code (Gitlab comments)

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
• Appropriate user and developer documentation (or not necessary)
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.
• Check: Are there specifications? Are they satisfied?

src/linkahead/common/models.py

@@ -1304,7 +1304,11 @@ class Entity:
                         elif v.name is not None:
                             v_elem.text = str(v.name)
                         else:
-                            v.to_xml(v_elem, visited_entities=visited_entities.copy())
+                            # We could consider reworking this to use value
+                            # instead of text analogously to scalar Entity
+                            # values, if this is supported server-side?
+                            dt_str = xml2str(v.to_xml(visited_entities=visited_entities.copy()))
+                            v_elem.text = dt_str
                     elif v == "":
                         v_elem.append(etree.Element("EmptyString"))
                     elif v is None: