fix(lmxproxy): protect probe subscription from ReadAsync teardown, add instance configs

ReadAsync internally subscribes/unsubscribes the same ScanTime tag used
by the persistent probe, which was tearing down the probe subscription
and triggering false reconnects every ~5s. Guard UnsubscribeInternal and
stored subscription state so the probe tag is never removed by other
callers. Also removes DetailedHealthCheckService (redundant with the
persistent probe), adds per-instance config files (appsettings.v2.json,
appsettings.v2b.json) loaded via LMXPROXY_INSTANCE env var so deploys
no longer overwrite port settings.

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

lmxproxy/src/ZB.MOM.WW.LmxProxy.Host/Program.cs

@@ -10,10 +10,12 @@ namespace ZB.MOM.WW.LmxProxy.Host
     {
         static int Main(string[] args)
         {
-            // 1. Build configuration
+            // 1. Build configuration (instance override file loaded from LMXPROXY_INSTANCE env var)
+            var instance = Environment.GetEnvironmentVariable("LMXPROXY_INSTANCE");
             var configuration = new ConfigurationBuilder()
                 .SetBasePath(AppDomain.CurrentDomain.BaseDirectory)
                 .AddJsonFile("appsettings.json", optional: false, reloadOnChange: false)
+                .AddJsonFile($"appsettings.{instance}.json", optional: true, reloadOnChange: false)
                 .AddEnvironmentVariables()
                 .Build();