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

@@ -1293,7 +1293,8 @@ class Entity:
                 elif self.value.name is not None:
                     xml.text = str(self.value.name)
                 else:
-                    self.value.to_xml(xml, visited_entities=visited_entities.copy())
+                    dt_str = xml2str(self.value.to_xml(visited_entities=visited_entities.copy()))
+                    xml.text = dt_str
             elif isinstance(self.value, list):
                 for v in self.value:
                     v_elem = etree.Element("Value")
@@ -1304,9 +1305,6 @@ class Entity:
                         elif v.name is not None:
                             v_elem.text = str(v.name)
                         else:
-                            # 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 == "":