🛡️ Sentinel: [CRITICAL/HIGH] Fix prototype pollution in AI extra body

[dttdrv]

🚨 Severity: CRITICAL
💡 Vulnerability: The mergeExtraBody function in backend/src/services/aiRouter.ts shallow-copied properties from a user-provided extraBody object into a payload object without checking for sensitive keys. This allowed attackers to potentially override prototype properties like __proto__, constructor, or prototype if the input payload contained these maliciously crafted properties.
🎯 Impact: Prototype pollution can lead to application logic bypass, unexpected behavior, or Denial of Service (DoS) attacks depending on how the polluted properties are subsequently used.
🔧 Fix: Added explicit blocklist checks for __proto__, constructor, and prototype in the loop that merges extraBody properties.
✅ Verification: Run npx vitest run and confirm all tests pass. Ad-hoc testing confirms prototype properties are not transferred to the target object.

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PR created automatically by Jules for task 16236615034221000408 started by @dttdrv

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