refactor: replace internal UnitOfWork with NotificationUnitOfWork and NotificationBaseRepository

- Consolidate unit of work implementation with NotificationUnitOfWork.
- Refactor repositories to use NotificationBaseRepository for consistency.
- Simplify request handlers by removing IUnitOfWork dependency.
- Update related tests and service registration.

HrynCo.NotificationService.Services/EmailChannels/TestSmtp/TestSmtpHandler.cs

@@ -1,6 +1,5 @@
 using System.Net;
 using System.Net.Mail;
-using HrynCo.DAL.Abstract;
 using HrynCo.NotificationService.Services.Core;
 using HrynCo.NotificationService.Services.Logging;
 using static HrynCo.NotificationService.Services.Core.ServiceResultHelper;
@@ -11,9 +10,8 @@ internal sealed class TestSmtpHandler
     : RequestHandler<TestSmtpCommand, ServiceResult<Unit>>
 {
     public TestSmtpHandler(
-        IContextualSerilogLogger<TestSmtpCommand> logger,
-        IUnitOfWork unitOfWork)
-        : base(logger, unitOfWork)
+        IContextualSerilogLogger<TestSmtpCommand> logger)
+        : base(logger)
     {
     }