Check if the `UNSPECIFIED` module-level sentinel is needed

Follow-up from PR #23558 review: `Unspecified()` already always returns a singleton, so it's
unclear whether the module-level `UNSPECIFIED` instance is still needed, or if `Unspecified()`
should just be used directly wherever a sentinel is required:

https://github.com/NVIDIA/cudf/blob/d4ecacec0cbdcb420a786416ff3b9e50111f0ca9/python/cudf_polars/cudf_polars/utils/config.py#L68-L93

## Related

- PR #23558
- https://github.com/NVIDIA/cudf/pull/23558#discussion_r3777718565

Read python/cudf_polars/cudf_polars/utils/config.py around lines 68–93, then search for UNSPECIFIED and Unspecified() references. Review PR #23558 and its linked discussion to determine which sentinel form is required; done means the choice is resolved and affected references are consistent.

Written by the indexing model from the issue text.