fix(dataset): resolve relative sample files on Windows and filesystem URIs (#4502)

[Anai-Guo]

Fixes #4502.

Problem

json_dataset() and csv_dataset() store Dataset.location with os.path.abspath(), while resolve_sample_files() derives the parent directory by splitting that value on the fsspec filesystem separator:

fs = filesystem(dataset.location)
parent_dir = fs.sep.join(dataset.location.split(fs.sep)[:-1])

Two things go wrong:

1. Windows — abspath() returns backslashes but LocalFileSystem.sep is /, so the split finds no separator, parent_dir becomes "", and the candidate path /images/ballons.png does not exist — the sample file is silently left relative.
2. Filesystem URIs — os.path.abspath() treats file:// / s3:// as local path strings and corrupts the location before any resolution happens.

Verification

Reproduced and verified on Windows 11 / Python 3.12 against the real package.

Before (both fail):

FAILED test_dataset_image_paths - AssertionError: assert WindowsPath('images/ballons.png').exists()
FAILED test_dataset_image_paths_file_uri - AssertionError
  location = 'D:\...\file:\D:\...\images.jsonl'   # <- URI corrupted by abspath()

After: both pass.

Fix

Use the filesystem-aware helpers that already exist in inspect_ai._util.file:

• absolute_file_path() for the location — it preserves URI schemes and only resolves genuinely relative local paths.
• dirname() for the parent directory — it is protocol- and separator-aware (verified against s3://b/sub/d.jsonl → s3://b/sub, file:///c/x/d.jsonl → file:///c/x, C:\x\d.jsonl → C:\x).

import os becomes unused in both readers and is removed.

Adds a regression test (test_dataset_image_paths_file_uri) covering a file:// dataset location; the pre-existing test_dataset_image_paths covers the Windows local-path case and now passes on Windows.

Verified no regression in the plain csv_dataset / json_dataset / file_dataset read paths.

🤖 Generated with Claude Code