🛡️ Sentinel: [CRITICAL] Fix timing attack vulnerability in A2A token validation

[google-labs-jules]

🚨 Severity: CRITICAL
💡 Vulnerability: The A2A token validation in the express middleware had a length check before calling crypto.timingSafeEqual. This created a short-circuit fast path that could leak the length of the secret token via timing differences.
🎯 Impact: An attacker could perform a timing attack to guess the length of the valid authorization token, which reduces the search space and might aid in bruteforcing the token.
🔧 Fix: Both the incoming token and the expected token are now hashed using SHA-256 before comparison. This ensures the inputs to timingSafeEqual are always the same length and evaluation is truly constant-time, eliminating the vulnerability while avoiding length mismatch errors.
✅ Verification: Verified using bun run test:full and by checking that tests for the A2A server pass.

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PR created automatically by Jules for task 16171780010719966223 started by @YoungSx

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