🛡️ Sentinel: [CRITICAL] Fix path traversal in ApiFilesGet

[thirdeyenation]

🚨 Severity: CRITICAL
💡 Vulnerability: Path traversal in api/api_files_get.py. User-provided paths starting with /a0/ were resolved directly to files.get_abs_path(relative_path) without any subsequent verification. This allowed directory traversal sequences (../) to escape the application's base directory and read arbitrary files on the system.
🎯 Impact: An attacker with API access could read sensitive files on the host (e.g., /etc/passwd, ssh keys, environment variables) by passing a carefully crafted relative path.
🔧 Fix: Added a security validation check using files.is_in_base_dir(external_path) before attempting to read the resolved file path. This ensures any file returned by the API strictly resides within the allowed base directory.
✅ Verification: Wrote local test scripts to mock the API request and confirmed that traversing to /etc/passwd correctly triggers the security check, returns a warning, and skips reading the file.

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PR created automatically by Jules for task 8998778357302992232 started by @thirdeyenation

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