feat: add test channel feature in admin UI

- POST /admin/channels/{id}/test — direct SMTP send via MailKit
- Test button shown only on existing channels (not create)
- Bootstrap modal with recipient email input and spinner
- Inline success/error result inside the modal

Ref: IT-628

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

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

@@ -5,8 +5,11 @@ 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;
 
 namespace HrynCo.NotificationService.Web.Controllers.Admin;
 
@@ -142,6 +145,46 @@ public class AdminChannelsController : Controller
         return RedirectToAction(nameof(Index));
     }
 
+    // POST /admin/channels/{id}/test
+    [HttpPost("{id:guid}/test")]
+    public async Task<IActionResult> Test(Guid id, [FromBody] TestChannelRequest request, CancellationToken ct)
+    {
+        var result = await _mediator.Send(new GetEmailChannelQuery(id), ct);
+        if (!result.IsSuccess || result.Result is null)
+            return NotFound(new { success = false, message = "Channel not found." });
+
+        if (result.Result.Settings is not SmtpChannelSettings smtp)
+            return BadRequest(new { success = false, message = "Only SMTP channels are supported." });
+
+        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")
+            {
+                Text = $"This is a test email sent from the Notification Service admin panel.\n\nChannel: {result.Result.ServiceName}\nHost: {smtp.Host}:{smtp.Port}"
+            };
+
+            using var client = new SmtpClient();
+            var secureSocket = smtp.UseSsl ? SecureSocketOptions.SslOnConnect : SecureSocketOptions.StartTlsWhenAvailable;
+            await client.ConnectAsync(smtp.Host, smtp.Port, secureSocket, ct);
+
+            if (!string.IsNullOrWhiteSpace(smtp.Username))
+                await client.AuthenticateAsync(smtp.Username, smtp.Password, ct);
+
+            await client.SendAsync(message, ct);
+            await client.DisconnectAsync(true, ct);
+
+            return Ok(new { success = true, message = $"Test email sent to {request.ToEmail}." });
+        }
+        catch (Exception ex)
+        {
+            return Ok(new { success = false, message = ex.Message });
+        }
+    }
+
     // POST /admin/channels/{id}/delete
     [HttpPost("{id:guid}/delete")]
     [ValidateAntiForgeryToken]
@@ -151,3 +194,5 @@ public class AdminChannelsController : Controller
         return RedirectToAction(nameof(Index));
     }
 }
+
+public record TestChannelRequest(string ToEmail);