🛡️ Sentinel: [HIGH] Fix prototype pollution in aiRouter

[dttdrv]

🚨 Severity: HIGH
💡 Vulnerability: The mergeExtraBody function in backend/src/services/aiRouter.ts shallow-merged user-supplied JSON from API requests directly into a system-constructed payload object. It iterated over properties without sanitizing standard prototype injection keys. An attacker could provide __proto__, constructor, or prototype in their request's extraBody, allowing them to mutate the prototype chain of the request payload or surrounding objects.
🎯 Impact: If exploited, this could lead to Denial of Service (DoS) or logic bypasses during AI API request construction and routing.
🔧 Fix: Added __proto__, constructor, and prototype to the explicitly blocked set of keys that are not copied over from the user's extraBody object into the request payload.
✅ Verification: Verified by executing the project's Vitest test suite (npx vitest run) to ensure no regressions. The change successfully blocks assignment of prototype keys during the iteration inside mergeExtraBody.

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PR created automatically by Jules for task 9122693872965937823 started by @dttdrv

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