Fix fallback model_fields class property

python/lsst/daf/butler/_compat.py

@@ -136,6 +136,7 @@ def model_rebuild(
                 return None
 
 else:
+    from astropy.utils.decorators import classproperty
 
     class _BaseModelCompat(BaseModel):  # type:ignore[no-redef]
         """Methods from pydantic v2 that can be used in pydantic v1."""
@@ -175,11 +176,15 @@ def model_dump_json(
 
         @classmethod  # type: ignore
         def model_construct(cls, _fields_set: set[str] | None = None, **values: Any) -> Self:
+            # BaseModel.construct() is very close to what we previously
+            # implemented manually in each direct() method but does have one
+            # extra loop in it to fill in defaults and handle aliases.
             return cls.construct(_fields_set=_fields_set, **values)
 
-        @property
-        def model_fields(self) -> dict[str, FieldInfo]:  # type: ignore
-            return self.__fields__  # type: ignore
+        @classmethod
+        @classproperty
+        def model_fields(cls) -> dict[str, FieldInfo]:  # type: ignore
+            return cls.__fields__
 
         @classmethod
         def model_rebuild(