Fix mypy validation error by adding explicit type guard function

Fixes:

python/lsst/daf/butler/queries/expression_factory.py:468: error: Argument "field" to "DatasetFieldReference" has incompatible type "str";
expected "Literal['dataset_id', 'ingest_date', 'run', 'collection', 'timespan']"  [arg-type]

triggered by pydantic 2.9.0.

python/lsst/daf/butler/queries/expression_factory.py

@@ -463,7 +463,7 @@ def __repr__(self) -> str:
     # to include Datastore record fields.
 
     def __getattr__(self, field: str) -> ScalarExpressionProxy:
-        if field not in tree.DATASET_FIELD_NAMES:
+        if not tree.is_dataset_field(field):
             raise AttributeError(field)
         expression = tree.DatasetFieldReference(dataset_type=self._dataset_type, field=field)
         return ResolvedScalarExpressionProxy(expression)

python/lsst/daf/butler/queries/tree/_base.py

@@ -32,10 +32,11 @@
     "ColumnExpressionBase",
     "DatasetFieldName",
     "DATASET_FIELD_NAMES",
+    "is_dataset_field",
 )
 
 from abc import ABC, abstractmethod
-from typing import TYPE_CHECKING, Any, ClassVar, Literal, TypeAlias, TypeVar, cast, get_args
+from typing import TYPE_CHECKING, Any, ClassVar, Literal, TypeAlias, TypeGuard, TypeVar, cast, get_args
 
 import pydantic
 
@@ -63,6 +64,22 @@
 _O = TypeVar("_O")
 
 
+def is_dataset_field(s: str) -> TypeGuard[DatasetFieldName]:
+    """Validate a field name.
+
+    Parameters
+    ----------
+    s : `str`
+        The field name to test.
+
+    Returns
+    -------
+    is_field : `bool`
+        Whether or not this is a dataset field.
+    """
+    return s in DATASET_FIELD_NAMES
+
+
 class QueryTreeBase(pydantic.BaseModel):
     """Base class for all non-primitive types in a query tree."""