🛡️ Sentinel: [HIGH] Fix DoS via Rate Limit Bypass

[arumes31]

🚨 Severity: HIGH
💡 Vulnerability: Denial of Service via Rate Limit Bypass. Fast-path rate limit handlers attempting to avoid reading large request bodies used a getRD(c) helper to fetch the redirect target. This helper, internally falling back to c.FormValue("rd"), implicitly parsed the entire request body before the rate limit logic was enforced, defeating the exhaustion protection.
🎯 Impact: An attacker could submit large, malformed payloads to early rate-limited endpoints (like login attempts) and still force the application to allocate memory and parse the body, leading to resource exhaustion and Denial of Service.
🔧 Fix: Replaced all usages of the getRD(c) helper with direct calls to c.QueryParam("rd") and removed the helper function entirely. This strictly restricts redirect extraction to query parameters, preventing implicit form parsing in error and fast-path handlers while templates (like login.html) continue to pass rd correctly in the URL.
✅ Verification: Verify that tests pass. The application now properly checks and enforces early rate limits before reading body content.

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PR created automatically by Jules for task 3809533184843826761 started by @arumes31

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