fix(kernels): prevent output files from escaping the target path

[sridipbasu]

Summary

While looking at the kaggle kernels output flow, I found that a server-provided file name could cause the CLI to write a file outside the directory given with --path.

For example, if the file name was ../outside.txt, the CLI would join it directly with the target directory and write the file outside that directory.

The same issue also happens with absolute paths. This means the CLI was trusting the file name returned by the server without checking that the final path was still inside the requested directory.

Fix

I added a path containment check before downloading the file.

The check uses the existing _is_within_directory helper and makes sure the resolved output path stays inside the requested --path.

If the file would escape the directory, the CLI now refuses to download it instead of writing the file.

Normal nested output paths such as sub/dir/result.csv still work as before.

Tests

I added regression tests for:

• ../outside.txt
• absolute paths
• valid nested paths

The tests also check that files with unsafe paths are rejected before they are fetched.

Ran:

pytest tests/unit/test_kernels_output_paths.py test_kernels_logs.py test_cli_kernels.py test_download_file_paths.py

pytest tests/unit

black --check

All unit tests pass: 1346 passed.

I also checked the new tests against the old implementation. The traversal and absolute-path tests failed before the fix and pass with the fix.