feat(lmxproxy): replace subscribe/unsubscribe health probe with persistent subscription

The old probe did a subscribe-read-unsubscribe cycle every 5 seconds to
check connection health. This created unnecessary churn and didn't detect
the failure mode where long-lived subscriptions silently stop receiving
COM callbacks (e.g. stalled STA message pump). The new approach keeps a
persistent subscription on the health check tag and forces reconnect if
no value update arrives within a configurable threshold (ProbeStaleThresholdMs,
default 5s). Also adds STA message pump debug logging (5-min heartbeat with
message counters) and fixes log file path resolution for Windows services.

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

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

@@ -34,9 +34,7 @@
 
   "HealthCheck": {
     "TestTagAddress": "DevPlatform.Scheduler.ScanTime",
-    "ProbeTimeoutMs": 5000,
-    "MaxConsecutiveTransportFailures": 3,
-    "DegradedProbeIntervalMs": 30000
+    "ProbeStaleThresholdMs": 5000
   },
 
   "ServiceRecovery": {
@@ -58,7 +56,8 @@
       "Override": {
         "Microsoft": "Warning",
         "System": "Warning",
-        "Grpc": "Information"
+        "Grpc": "Information",
+        "ZB.MOM.WW.LmxProxy.Host.MxAccess.StaComThread": "Debug"
       }
     },
     "WriteTo": [