fix(historian-gateway): dispose alarm-write channel at shutdown + ServerHistorian startup diagnostic

Claude-Session: https://claude.ai/code/session_012SDSQ3AcaXqPcBtDESBRii

src/Server/ZB.MOM.WW.OtOpcUa.Host/Program.cs

@@ -109,6 +109,8 @@ if (hasDriver)
     var serverHistorianOptions = builder.Configuration
         .GetSection(ServerHistorianOptions.SectionName).Get<ServerHistorianOptions>()
         ?? new ServerHistorianOptions();
+    foreach (var warning in serverHistorianOptions.Validate())
+        Log.Warning("ServerHistorian misconfiguration detected at startup: {Warning}", warning);
     builder.Services.AddAlarmHistorian(
         builder.Configuration,
         (_, sp) => GatewayHistorian.CreateAlarmWriter(serverHistorianOptions, sp));