🛡️ Sentinel: [CRITICAL/HIGH] Fix Prototype Pollution vulnerability in mergeExtraBody

[dttdrv]

🚨 Severity: CRITICAL/HIGH
💡 Vulnerability: The mergeExtraBody function in backend/src/services/aiRouter.ts performs a shallow merge of user-provided JSON (extraBody) into a payload object. It did not block keys like __proto__, constructor, or prototype, allowing an attacker to modify the prototype of the target object.
🎯 Impact: This could lead to a prototype pollution vulnerability, which might be leveraged for denial of service or altering the runtime behavior of the backend logic.
🔧 Fix: Updated the blocked set in mergeExtraBody to include __proto__, constructor, and prototype, ensuring these keys are never copied during the object merge.
✅ Verification: Ensure the test suite continues to pass (npx vitest run) and that the modified .jules/sentinel.md journal accurately reflects this critical learning.

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PR created automatically by Jules for task 13412445438366289839 started by @dttdrv

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