Add XML documentation across gateway, worker, and .NET client

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

@@ -2,6 +2,7 @@ namespace MxGateway.Client.Tests;
 
 public sealed class MxGatewayClientOptionsTests
 {
+    /// <summary>Verifies that options with valid endpoint and API key pass validation.</summary>
     [Fact]
     public void Validate_WithAbsoluteEndpointAndApiKey_Succeeds()
     {
@@ -14,6 +15,7 @@ public sealed class MxGatewayClientOptionsTests
         options.Validate();
     }
 
+    /// <summary>Verifies that empty API key causes validation to fail.</summary>
     [Fact]
     public void Validate_WithEmptyApiKey_Throws()
     {
@@ -26,6 +28,7 @@ public sealed class MxGatewayClientOptionsTests
         Assert.Throws<ArgumentException>(options.Validate);
     }
 
+    /// <summary>Verifies that invalid retry options cause validation to fail.</summary>
     [Fact]
     public void Validate_WithInvalidRetryOptions_Throws()
     {