fix: add an infinite recursion guard to the post model validator

The unsetting of fields leads to a retrigger of the model validation,
where the sync logic ends up in an infinite recursion. Add a flag so
that it runs only once and prevents reentry.

Author: schivmeister

src/ted_sws/core/model/transform.py

@@ -191,7 +191,12 @@ def sync_legacy_and_mssdk_fields(self) -> 'MappingPackage':
 
         This ensures the model works with both old code using legacy fields
         and new code using MSSDK v2 structure.
+        Prevents infinite recursion by using a private _sync_done flag.
         """
+        if getattr(self, "_sync_done", False):
+            return self
+        setattr(self, "_sync_done", True)
+
         # If MSSDK v2 fields are missing but legacy fields exist, populate from legacy
         # FIXME: this is a transitional solution for code where the legacy file system package parsing is done
         if self.metadata is None: