typing: accept sequences for Dataset file loaders

[biefan]

Summary

• update Dataset.from_csv, from_json, from_parquet, and from_text type hints from list[PathLike] to Sequence[PathLike]
• normalize non-string sequences to lists before passing paths to readers
• add path-type tests to cover tuple input for all four loaders

Why

list is invariant for static type checkers, so list[str] can be rejected against list[PathLike] even though str itself is valid path input. Sequence[PathLike] is covariant and better matches real usage.

Validation

• uv run --python 3.11 --with-editable . --with pytest --with setuptools --with absl-py -m pytest tests/test_arrow_dataset.py::test_dataset_from_csv_path_type tests/test_arrow_dataset.py::test_dataset_from_json_path_type tests/test_arrow_dataset.py::test_dataset_from_parquet_path_type tests/test_arrow_dataset.py::test_dataset_from_text_path_type -q
• uv run --python 3.11 --with ruff --with setuptools ruff check src/datasets/arrow_dataset.py tests/test_arrow_dataset.py

Fixes #5354