Restore _legacy_no_dict_keys_sorting so _check_legacy_cache2 can find 2.15.0 caches.

[cpruijsen]

Summary

Restore the early return in Pickler._batch_setitems that honours _legacy_no_dict_keys_sorting, keeping the *args / **kwargs passthrough from #7817. #7817 updated Pickler._batch_setitems for Python 3.14's extra argument and, in the same hunk, dropped that return. _check_legacy_cache2 still patches _legacy_no_dict_keys_sorting to recompute the config_id datasets 2.15.0 would have written, so since 4.4.0 the lookup hashes dicts with sorted keys and misses old cache directories when data_files keys are not already sorted ({"train": ..., "test": ...}).

With _legacy_no_dict_keys_sorting set, Hasher.hash({"train": ["train.csv"], "test": ["test.csv"]}) is again 711511d8f1d9bc25, the value measured from 2.15.0 (issue author, Python 3.11) and reproduced here on 3.12. Default hashing is unchanged: the same two dicts still hash equal without the flag.

Chose option 1 on #8410: restore the reader rather than delete the 2.14/2.15 compat path (option 2). That is what #6514 implemented and is two lines to revert. Can drop _check_legacy_cache2 / the flag instead if you would rather retire that layout.

Fixes #8410

Test plan

• tests/test_fingerprint.py::HashingTest::test_hash_dict_legacy_no_dict_keys_sorting fails without the early return (cfd3b51f0f8e9fd8 == cfd3b51f0f8e9fd8) and passes with it
• Default (unpatched) hashing of the same two dicts remains equal
• Neighbouring HashingTest cases that do not need optional extras: 9 passed, 6 deselected
• Optional: load a dataset whose 2.15.0 cache dir used dict-valued data_files with unsorted keys and confirm builder.cache_dir points at that directory