refactor: replace mailkit with system.net.mail for smtp test

MailKit/MimeKit are unnecessary third-party dependencies with known
vulnerabilities. .NET's built-in System.Net.Mail.SmtpClient handles
the same test send without any additional packages.

Ref: IT-628

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

HrynCo.NotificationService.Web/Controllers/Admin/AdminChannelsController.cs

@@ -5,11 +5,10 @@ using HrynCo.NotificationService.Services.EmailChannels.Get;
 using HrynCo.NotificationService.Services.EmailChannels.GetAll;
 using HrynCo.NotificationService.Services.EmailChannels.Update;
 using HrynCo.NotificationService.Web.Controllers.Admin.ViewModels;
-using MailKit.Net.Smtp;
-using MailKit.Security;
 using MediatR;
 using Microsoft.AspNetCore.Mvc;
-using MimeKit;
+using System.Net;
+using System.Net.Mail;
 
 namespace HrynCo.NotificationService.Web.Controllers.Admin;
 
@@ -153,28 +152,24 @@ public class AdminChannelsController : Controller
 
         try
         {
-            var message = new MimeMessage();
-            message.From.Add(new MailboxAddress(smtp.FromName, smtp.FromEmail));
-            message.To.Add(MailboxAddress.Parse(request.ToEmail));
-            message.Subject = "✅ Test email from Notification Service";
-            message.Body = new TextPart("plain")
+            using var client = new SmtpClient(smtp.Host, smtp.Port)
             {
-                Text = $"This is a test email sent from the Notification Service admin panel.\n\nChannel: {result.Result.ServiceName}\nHost: {smtp.Host}:{smtp.Port}"
+                EnableSsl = smtp.UseSsl,
+                Credentials = string.IsNullOrWhiteSpace(smtp.Username)
+                    ? null
+                    : new NetworkCredential(smtp.Username, smtp.Password)
             };
 
-            using var client = new SmtpClient();
-            var secureSocket = smtp.Port == 465
-                ? SecureSocketOptions.SslOnConnect
-                : smtp.UseSsl
-                    ? SecureSocketOptions.StartTls
-                    : SecureSocketOptions.None;
-            await client.ConnectAsync(smtp.Host, smtp.Port, secureSocket, ct);
+            var message = new MailMessage
+            {
+                From = new MailAddress(smtp.FromEmail, smtp.FromName),
+                Subject = "✅ Test email from Notification Service",
+                Body = $"This is a test email sent from the Notification Service admin panel.\n\nChannel: {result.Result.ServiceName}\nHost: {smtp.Host}:{smtp.Port}",
+                IsBodyHtml = false
+            };
+            message.To.Add(request.ToEmail);
 
-            if (!string.IsNullOrWhiteSpace(smtp.Username))
-                await client.AuthenticateAsync(smtp.Username, smtp.Password, ct);
-
-            await client.SendAsync(message, ct);
-            await client.DisconnectAsync(true, ct);
+            await client.SendMailAsync(message, ct);
 
             return Ok(new { success = true, message = $"Test email sent to {request.ToEmail}." });
         }