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.Services/EmailChannels/TestSmtp/TestSmtpCommand.cs

@@ -0,0 +1,18 @@
+using HrynCo.NotificationService.Services.Core;
+using MediatR;
+
+namespace HrynCo.NotificationService.Services.EmailChannels.TestSmtp;
+
+/// <summary>
+/// Sends a test email using the provided SMTP settings without persisting anything.
+/// </summary>
+public sealed record TestSmtpCommand(
+    string Host,
+    int Port,
+    string Username,
+    string Password,
+    bool UseSsl,
+    string FromEmail,
+    string FromName,
+    string ToEmail
+) : IRequest<ServiceResult<Core.Unit>>;