Add Polly resilience policies

clients/dotnet/MxGateway.Client.Tests/MxGatewayClientOptionsTests.cs

@@ -25,4 +25,17 @@ public sealed class MxGatewayClientOptionsTests
 
         Assert.Throws<ArgumentException>(options.Validate);
     }
+
+    [Fact]
+    public void Validate_WithInvalidRetryOptions_Throws()
+    {
+        var options = new MxGatewayClientOptions
+        {
+            Endpoint = new Uri("http://localhost:5000"),
+            ApiKey = "test-api-key",
+            Retry = new MxGatewayClientRetryOptions { MaxAttempts = 0 },
+        };
+
+        Assert.Throws<ArgumentOutOfRangeException>(options.Validate);
+    }
 }