chore(lmxproxy): switch health probe tag to DevAppEngine.Scheduler.ScanTime, remove temp prompts

AppEngine built-in tag is always present and constantly updating (~1s),
making it a more reliable probe than a user-deployed TestChildObject tag.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lmxproxy/src/ZB.MOM.WW.LmxProxy.Host/appsettings.json

@@ -33,7 +33,7 @@
   },
 
   "HealthCheck": {
-    "TestTagAddress": "TestChildObject.TestBool",
+    "TestTagAddress": "DevAppEngine.Scheduler.ScanTime",
     "ProbeTimeoutMs": 5000,
     "MaxConsecutiveTransportFailures": 3,
     "DegradedProbeIntervalMs": 30000