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

[dttdrv]

🚨 Severity: HIGH
💡 Vulnerability: The mergeExtraBody function directly assigned keys from a user-provided JSON object (extraBody) onto payload. Although it skipped some restricted keys like model, it did not sanitize against __proto__, constructor, or prototype, leaving it vulnerable to prototype pollution if malicious input is supplied.
🎯 Impact: Attackers could manipulate prototype chains leading to application crashes, logic bypasses, or potential remote code execution (RCE) depending on how properties are accessed elsewhere.
🔧 Fix: Explicitly added "__proto__", "constructor", and "prototype" to the blocked set in mergeExtraBody before merging properties.
✅ Verification: Ran npx vitest run tests and confirmed everything functions as expected with the mitigation in place. Code review also verified this is an industry standard and completely safe practice.

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PR created automatically by Jules for task 15947103853043525928 started by @dttdrv

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