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

@@ -1270,8 +1270,7 @@ class Entity:
         if visited_entities is None:
             visited_entities = []
         if self in visited_entities:
-            xml.text = "..."
-            return xml
+            return etree.Comment("Recursive reference")
         visited_entities.append(self)
 
         # unwrap wrapped entity