feat(TestBase): seed real email templates in TestBase setup

- Added `RealEmailTemplates` property with predefined email templates.
- Populated the repository with these templates in the `Setup` method.
- Ensures tests have consistent, realistic email data available.

EnvelopeGenerator.Application/Common/Notifications/SendMailHandler.cs

@@ -99,8 +99,8 @@ public abstract class SendMailHandler<TNotification> : INotificationHandler<TNot
         var placeHolders = CreatePlaceHolders(notification);
 
         var temp = await TempRepo
-            .ReadOnly()
-            .SingleOrDefaultAsync(x => x.Name == notification.TemplateType.ToString(), cancel)
+            .Where(x => x.Name == notification.TemplateType.ToString())
+            .SingleOrDefaultAsync(cancel)
                 ?? throw new InvalidOperationException($"Receiver information is missing in the notification." +
                 $"{typeof(TNotification)}:\n {notification.ToJson(Format.Json.ForDiagnostics)}");