Dataset relative files fail on Windows and filesystem URIs

[ShiroKSH]

Summary

json_dataset() and csv_dataset() do not reliably resolve relative sample files on Windows or when the dataset path is a filesystem URI.

On Windows, the existing tests/dataset/test_dataset.py::test_dataset_image_paths test fails on current main because images/ballons.png remains relative instead of resolving beside images.jsonl.

Root cause

The readers store Dataset.location with os.path.abspath(), while resolve_sample_files() derives the parent by splitting that value on the fsspec filesystem separator.

• Windows abspath() produces backslashes, while LocalFileSystem.sep is /, so the split does not find the parent directory.
• os.path.abspath() treats file://, s3://, and other filesystem URIs as local path strings, corrupting the stored location before relative files are resolved.

Reproduction

uv run pytest tests/dataset/test_dataset.py::test_dataset_image_paths -vv

Current result on Windows:

AssertionError: assert WindowsPath('images/ballons.png').exists()

Expected behavior

Dataset locations should retain their filesystem scheme, and relative sample files should resolve against the dataset's parent directory on local and remote filesystems.

I can open a focused PR using the repository's filesystem-aware path helpers, with regression coverage for local paths and filesystem URIs.

[ShiroKSH]

I'd like to work on this. I'll keep the change scoped to filesystem-aware location normalization and focused regression tests.