Enforce boolean ascending for dask-cudf sort_values (#9814)

It is possible to pass a list of `ascending` booleans to dask-cudf's `sort_values`, which is not yet supported by `quantile_divisions` (which computes divisions for all sort-by columns sorted in ascending order), and can cause undefined behavior.

This small PR adds a check that `ascending` is a boolean before computing quantile divisions; note that this check happens _after_ the single-partition case is handled, as cuDF can handle lists of `ascending` booleans.

Authors:
  - Charles Blackmon-Luca (https://github.com/charlesbluca)

Approvers:
  - GALI PREM SAGAR (https://github.com/galipremsagar)

URL: #9814

python/dask_cudf/dask_cudf/sorting.py

@@ -224,6 +224,8 @@ def sort_values(
     na_position="last",
 ):
     """Sort by the given list/tuple of column names."""
+    if not isinstance(ascending, bool):
+        raise ValueError("ascending must be either True or False")
     if na_position not in ("first", "last"):
         raise ValueError("na_position must be either 'first' or 'last'")