fix(storage/artifacts): block path traversal via artifact_path

[Sp1kyss]

Summary

Both FilesystemArtifactStorage and S3ArtifactStorage joined attacker-controlled artifact_path values to a fixed prefix using pathlib.Path / artifact_path. Two pathlib quirks made that join unsafe:

• pathlib.Path('/a') / '/etc/passwd' returns /etc/passwd (the / operator silently discards the prefix when the right operand is absolute).
• pathlib.Path('/a') / '../etc/passwd' returns /a/../etc/passwd, which the OS resolves to /etc/passwd once the value is passed to shutil.copy / shutil.rmtree / boto3.

The SDK exposes Run.log_artifact(path, name=...), where name is propagated unchanged to upload_artifact(..., artifact_path=name), so a malicious caller can pick the destination filename and escape the configured artifact root.

Impact

• Arbitrary file overwrite via upload_artifact -> shutil.copy (e.g. dropping a file into /etc/cron.d/, /var/www/html/, or any path the Aim process can write to).
• Arbitrary directory deletion via delete_artifact -> shutil.rmtree.
• Arbitrary S3 key write / delete outside the configured prefix.

This is a distinct code path from previously-reported issues:

• CVE-2024-6396 -- _backup_run path traversal.
• CVE-2024-8769 -- LockManager.release_locks traversal.
• The previously-reported restore_run_backup tar extraction issue.

Fix

• Add safe_join (filesystem) and safe_join_posix (S3 keys) helpers in aim/storage/artifacts/artifact_storage.py. Both reject absolute paths, drive letters, and any .. segments. safe_join additionally re-validates that the resolved filesystem path stays inside the prefix.
• Switch FilesystemArtifactStorage and S3ArtifactStorage to use the helpers for upload, download, and delete.
• Add regression tests covering absolute paths, embedded .., delete safety, and a legitimate relative-path case.

Reproducer (before this PR)

`python
from aim.storage.artifacts.filesystem_storage import FilesystemArtifactStorage

storage = FilesystemArtifactStorage('file:///tmp/artifacts')

Writes attacker-controlled content to /etc/cron.d/aim_pwn:

storage.upload_artifact('/path/to/payload', '/etc/cron.d/aim_pwn')

Or via the SDK:

run.log_artifact('/path/to/payload', name='/etc/cron.d/aim_pwn')

`

After this PR the same call raises ValueError before any I/O happens.

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