fix(ci): set correct rate_limit_mode field in integration test security config

- The rate-limit integration test was sending rate_limit_enable:true in the
  security config POST, but the backend injects the Caddy rate_limit handler
  only when rate_limit_mode is the string "enabled"
- Because rate_limit_mode was absent from the payload, the database default
  of "disabled" persisted and the guard condition always evaluated false,
  leaving the handler uninjected across all 10 verify attempts
- Replaced the boolean rate_limit_enable with the string field
  rate_limit_mode:"enabled" to match the exact contract the backend enforces

scripts/rate_limit_integration.sh

@@ -262,7 +262,7 @@ SEC_CFG_PAYLOAD=$(cat <<EOF
 {
     "name": "default",
     "enabled": true,
-    "rate_limit_enable": true,
+    "rate_limit_mode": "enabled",
     "rate_limit_requests": ${RATE_LIMIT_REQUESTS},
     "rate_limit_window_sec": ${RATE_LIMIT_WINDOW_SEC},
     "rate_limit_burst": ${RATE_LIMIT_BURST},