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

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

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway/GatewayAlarmHistorianWriter.cs

@@ -36,7 +36,7 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway;
 ///         event forever.
 ///     </para>
 /// </remarks>
-public sealed class GatewayAlarmHistorianWriter : IAlarmHistorianWriter
+public sealed class GatewayAlarmHistorianWriter : IAlarmHistorianWriter, IDisposable
 {
     private readonly IHistorianGatewayClient _client;
     private readonly ILogger<GatewayAlarmHistorianWriter> _logger;
@@ -154,4 +154,11 @@ public sealed class GatewayAlarmHistorianWriter : IAlarmHistorianWriter
         // Unknown/unclassified gRPC code → dead-letter to avoid an infinite drain loop.
         _ => HistorianWriteOutcome.PermanentFail,
     };
+
+    /// <summary>
+    ///     Disposes the underlying gateway client and its gRPC channel. The concrete
+    ///     <see cref="HistorianGatewayClientAdapter"/> implements <see cref="IDisposable"/>; test doubles
+    ///     that only implement <see cref="IAsyncDisposable"/> are safely no-opped by the cast guard.
+    /// </summary>
+    public void Dispose() => (_client as IDisposable)?.Dispose();
 }