BUG: fix sort_index AssertionError with RangeIndex and level parameter

[repeating]

closes #64383

RangeIndex.sort_values(return_indexer=True) was returning a RangeIndex object as the indexer, but the block manager (managers.py) asserts isinstance(indexer, np.ndarray) before calling .take(). this caused sort_index(level=...) to crash with AssertionError any time the index stayed a RangeIndex (e.g. an already-sorted range index, or a default index with .names set).

the base class Index.sort_values always returns np.arange(..., dtype=np.intp) as the indexer — RangeIndex.sort_values was overriding for performance but breaking that contract. the fix is to return np.arange(len(self), dtype=np.intp) (or the reversed equivalent) instead of RangeIndex(rng).

added a regression test covering both reported cases, and a whatsnew entry under v3.0.2.

[jbrockmendel]

what are the consequences of this? knowing you have a RangeIndex can be good for performance downstream

[repeating]

the indexer here is only consumed by self._mgr.take(indexer, ...) which asserts isinstance(indexer, np.ndarray) — nothing downstream of sort_values(return_indexer=True) uses the indexer as a RangeIndex, it's purely a positional array for the block manager's take path. the base class Index.sort_values already returns np.ndarray for this, so this just brings the RangeIndex override in line with that contract.