fix(historian-gateway): dispose recorder value-writer channel + clearer OutboxPath warning

Addresses T18 review: GatewayHistorianValueWriter is a DI singleton holding a gRPC
channel — make it IAsyncDisposable so the container closes the channel gracefully at
shutdown. Tighten the blank-OutboxPath warning to state startup will fail.

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

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

@@ -25,7 +25,7 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway.Recorder;
 ///         durably queued must not be re-drained.
 ///     </para>
 /// </remarks>
-public sealed class GatewayHistorianValueWriter : IHistorianValueWriter
+public sealed class GatewayHistorianValueWriter : IHistorianValueWriter, IAsyncDisposable
 {
     private readonly IHistorianGatewayClient _client;
     private readonly ILogger<GatewayHistorianValueWriter> _logger;
@@ -86,4 +86,10 @@ public sealed class GatewayHistorianValueWriter : IHistorianValueWriter
             return false;
         }
     }
+
+    /// <summary>
+    ///     Disposes the underlying gateway client (and its gRPC channel). The DI container owns this
+    ///     writer as a singleton, so this fires once at host shutdown — closing the channel gracefully.
+    /// </summary>
+    public ValueTask DisposeAsync() => _client.DisposeAsync();
 }