Fix LogicalColumn mypy error

Per Jim, Timespan columns are not expected at this point in the code and would have been throwing an error already if they did show up.

python/lsst/daf/butler/registry/queries/butler_sql_engine.py

@@ -191,14 +191,16 @@ def to_payload(self, relation: Relation) -> sql.Payload[LogicalColumn]:
                 # contains (at the same time, since they have the same columns,
                 # aside from the special 'rownum' window-function column).
                 search_columns = self.extract_mapping(target.columns, search.columns)
-                partition_by = [search_columns[tag] for tag in operation.dimensions]
+                partition_by = [
+                    _assert_column_is_directly_usable_by_sqlalchemy(search_columns[tag])
+                    for tag in operation.dimensions
+                ]
                 row_number = sqlalchemy.sql.func.row_number()
+                rank_column = _assert_column_is_directly_usable_by_sqlalchemy(search_columns[operation.rank])
                 if partition_by:
-                    rownum_column = row_number.over(
-                        partition_by=partition_by, order_by=search_columns[operation.rank]
-                    )
+                    rownum_column = row_number.over(partition_by=partition_by, order_by=rank_column)
                 else:
-                    rownum_column = row_number.over(order_by=search_columns[operation.rank])
+                    rownum_column = row_number.over(order_by=rank_column)
                 window = self.select_items(
                     search_columns.items(), search, rownum_column.label("rownum")
                 ).subquery(f"{operation.rank.dataset_type}_window")
@@ -209,3 +211,13 @@ def to_payload(self, relation: Relation) -> sql.Payload[LogicalColumn]:
                 )
             case _:
                 return super().to_payload(relation)
+
+
+def _assert_column_is_directly_usable_by_sqlalchemy(column: LogicalColumn) -> sqlalchemy.sql.ColumnElement:
+    """Narrow a `LogicalColumn` to a SqlAlchemy ColumnElement, to satisfy the
+    typechecker in cases where no timespans are expected.
+    """
+    if isinstance(column, TimespanDatabaseRepresentation):
+        raise TypeError("Timespans not expected here.")
+
+    return column