docs: add missing XML doc comments across gateway, worker, and .NET client

Resolves 1113 documentation-completeness gaps flagged by CommentChecker
(MissingReturns, MissingInheritDoc, InheritDocMisused, MissingDoc,
MissingParam, RedundantInheritDoc) so the API surface is fully documented
and the analyzer scan is clean. Doc comments only; no code changes.

clients/dotnet/ZB.MOM.WW.MxGateway.Client/MxGatewayClient.cs

@@ -249,6 +249,7 @@ public sealed class MxGatewayClient : IAsyncDisposable
     /// <summary>
     /// Disposes the client and releases all resources.
     /// </summary>
+    /// <returns>A task that represents the asynchronous dispose operation.</returns>
     public ValueTask DisposeAsync()
     {
         if (_disposed)
@@ -318,6 +319,9 @@ public sealed class MxGatewayClient : IAsyncDisposable
     private static HttpMessageHandler CreateHttpHandler(MxGatewayClientOptions options) =>
         CreateHttpHandlerForTests(options);
 
+    /// <summary>Creates an <see cref="HttpMessageHandler"/> configured from the provided options for test use.</summary>
+    /// <param name="options">Client options used to configure TLS and timeouts.</param>
+    /// <returns>The configured HTTP message handler.</returns>
     internal static SocketsHttpHandler CreateHttpHandlerForTests(MxGatewayClientOptions options)
     {
         SocketsHttpHandler handler = new()