feat: add test button to create channel form using ad-hoc smtp test endpoint

- Add TestSmtpCommand and TestSmtpHandler for ad-hoc smtp testing without saving
- Add POST /admin/channels/test-smtp endpoint accepting raw smtp settings
- Show Test button on both Create and Edit forms
- Test reads current form values so channel can be tested before saving

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

@@ -5,6 +5,7 @@ using HrynCo.NotificationService.Services.EmailChannels.Delete;
 using HrynCo.NotificationService.Services.EmailChannels.Get;
 using HrynCo.NotificationService.Services.EmailChannels.GetUsageSummary;
 using HrynCo.NotificationService.Services.EmailChannels.Send;
+using HrynCo.NotificationService.Services.EmailChannels.TestSmtp;
 using HrynCo.NotificationService.Services.EmailChannels.Update;
 using HrynCo.NotificationService.Web.Controllers.Admin.ViewModels;
 using MediatR;
@@ -133,6 +134,20 @@ public class AdminChannelsController(IMediator mediator) : Controller
         return RedirectToAction(nameof(Index));
     }
 
+    // POST /admin/channels/test-smtp
+    [HttpPost("test-smtp")]
+    public async Task<IActionResult> TestSmtp([FromBody] TestSmtpRequest request, CancellationToken ct)
+    {
+        var result = await mediator.Send(new TestSmtpCommand(
+            request.Host, request.Port, request.Username, request.Password,
+            request.UseSsl, request.FromEmail, request.FromName, request.ToEmail), ct);
+
+        if (!result.IsSuccess)
+            return Ok(new { success = false, message = result.Error?.Message });
+
+        return Ok(new { success = true, message = $"Test email sent to {request.ToEmail}." });
+    }
+
     // POST /admin/channels/{id}/test
     [HttpPost("{id:guid}/test")]
     public async Task<IActionResult> Test(Guid id, [FromBody] TestChannelRequest request, CancellationToken ct)
@@ -165,3 +180,6 @@ public class AdminChannelsController(IMediator mediator) : Controller
 }
 
 public record TestChannelRequest(string ToEmail);
+public record TestSmtpRequest(
+    string Host, int Port, string Username, string Password,
+    bool UseSsl, string FromEmail, string FromName, string ToEmail);