fix(discovery): recover an nmap scan that succeeded despite a benign stderr warning

node-nmap (the vendored library, not our code) treats ANY stderr
output from the nmap binary as a fatal scan error -- including nmap's
own harmless RTT-calibration warning ("RTTVAR has grown to over N
seconds, decreasing to M"), which it prints *during* a scan that goes
on to complete normally. That meant a real, successful scan (valid XML
already sitting in the library's rawData) got discarded and reported
as a failed run with zero hosts discovered -- not just log noise as
initially assumed.

Recover in our own plugin code by detecting this specific known-benign
message and manually re-running node-nmap's own XML-parse-then-complete
path when there's actually output to parse. A genuine parse failure or
any other error message still rejects exactly as before -- this only
widens the recovery path.

nodejs/package.json

@@ -11,7 +11,7 @@
   "scripts": {
     "start": "node ./bin/www",
     "dev": "npx nodemon --ignore public/ ./bin/www",
-    "test": "NODE_ENV=test jest tests/groups.test.js tests/subtypes.test.js tests/site_join.test.js tests/site_config.test.js tests/site_replicate.test.js tests/proxy_client.test.js --forceExit"
+    "test": "NODE_ENV=test jest tests/groups.test.js tests/subtypes.test.js tests/site_join.test.js tests/site_config.test.js tests/site_replicate.test.js tests/proxy_client.test.js tests/nmap_plugin.test.js --forceExit"
   },
   "jest": {
     "testEnvironment": "node",