🛡️ Sentinel: [HIGH] Fix path traversal in file_browser.py

[thirdeyenation]

🚨 Severity: HIGH
💡 Vulnerability: Path traversal logic bypass when using string startswith on absolute paths without checking for os.sep or correctly checking path containment.
🎯 Impact: A malicious user could potentially read, delete, or write files in sibling directories of base_dir by exploiting the path prefix check.
🔧 Fix: Added a secure _is_in_base_dir method to FileBrowser that correctly checks for path containment using os.path.abspath and os.sep.
✅ Verification: Ensure tests pass and test with paths like /tmp/app_log when base_dir is /tmp/app.

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PR created automatically by Jules for task 6994022533431374378 started by @thirdeyenation

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