Fix Python 3.14 dict pickling compatibility - #8651

Closes #8373.

Summary

• passes the new Python 3.14 _batch_setitems object argument when the custom datasets pickler is called through the old two-argument path
• keeps the existing key sorting behavior for stable fingerprints
• adds a regression test that simulates the Python 3.14 parent pickler signature

Validation

• python -m compileall src\datasets\utils\_dill.py tests\test_fingerprint.py
• git diff --check

I could not run pytest or ruff locally because they are not installed in this environment.