🛡️ Sentinel: [CRITICAL/HIGH] Fix timing attack vulnerability in token validation

[google-labs-jules]

🚨 Severity: HIGH
💡 Vulnerability: A timing attack vulnerability in Bearer token validation where checking tokenBuffer.length === authTokenBuffer.length creates a short-circuit fast path, leaking the length of the secret token.
🎯 Impact: An attacker can deduce the length of the authentication token by measuring response times, significantly reducing the search space for brute-forcing the token.
🔧 Fix: Hashed both the provided token and the expected token using SHA-256 before comparison to ensure true constant-time evaluation and avoid leaking token length.
✅ Verification: Ran unit tests for the CLI serve command and the full test suite to ensure token validation functions properly and securely.

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PR created automatically by Jules for task 12217375062568890813 started by @YoungSx

[google-labs-jules]

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