Fix runtime review findings

clients/dotnet/MxGateway.Client/MxGatewayClientOptions.cs

@@ -23,6 +23,8 @@ public sealed class MxGatewayClientOptions
 
     public TimeSpan? StreamTimeout { get; init; }
 
+    public int MaxGrpcMessageBytes { get; init; } = 16 * 1024 * 1024;
+
     public MxGatewayClientRetryOptions Retry { get; init; } = new();
 
     public ILoggerFactory? LoggerFactory { get; init; }
@@ -66,6 +68,13 @@ public sealed class MxGatewayClientOptions
                 "The stream timeout must be greater than zero when configured.");
         }
 
+        if (MaxGrpcMessageBytes <= 0)
+        {
+            throw new ArgumentOutOfRangeException(
+                nameof(MaxGrpcMessageBytes),
+                "The maximum gRPC message size must be greater than zero.");
+        }
+
         if (UseTls && Endpoint.Scheme != Uri.UriSchemeHttps)
         {
             throw new ArgumentException(