Fix SampleDataset cache cleanup

[DarylOkeke]

Closing a SampleDataset could delete reusable task cache files. This tracks whether the dataset owns its directory, so close() only removes temporary data and keeps persistent or shared caches intact.

[fbonc]

in tests/core/test_caching.py:172, the test suite fails cause this test still expects the processed sample cache from BaseDataset.set_task() to be deleted when its context exits.

this PR intentionally preserves that reusable cache, so index.json and schema.pkl still exist. fix is to update these assertions to expect both files to remain.

[fbonc]

otherwise looks good.