🛡️ Sentinel: [CRITICAL] Fix path traversal in file_info and download_work_dir_file

[thirdeyenation]

🚨 Severity: CRITICAL
💡 Vulnerability: The get_abs_path function returns the absolute path untouched if the input is already absolute. This allowed path traversal to read arbitrary files via api/file_info.py and api/download_work_dir_file.py (e.g. providing /etc/passwd).
🎯 Impact: Attackers could read arbitrary sensitive files on the system or server.
🔧 Fix: Implemented files.is_in_base_dir(abs_path) boundary checks in api/file_info.py, api/download_work_dir_file.py, and helpers/file_browser.py to prevent access to files outside of the configured base directory.
✅ Verification: Tested against standard inputs and absolute paths. The path traversal attempt now logs a warning and raises an Exception or returns exists: False, resolving the vulnerability. Run pytest tests/ to confirm.

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PR created automatically by Jules for task 96874249595781153 started by @thirdeyenation

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