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

src/MxGateway.Server/Security/Authentication/ApiKeySecretGenerator.cs

@@ -2,8 +2,10 @@ using System.Security.Cryptography;
 
 namespace MxGateway.Server.Security.Authentication;
 
+/// <summary>Generates cryptographically secure API key secrets.</summary>
 public static class ApiKeySecretGenerator
 {
+    /// <summary>Generates a new random API key secret string.</summary>
     public static string Generate()
     {
         Span<byte> bytes = stackalloc byte[32];