feat: add MediatR handlers for template and channel CRUD

- ServiceResult<T>, ServiceError, ServiceErrorCode, Unit, ServiceResultHelper in Services/Core
- RequestHandler<TRequest, TResponse> base class (MediatR-adapted, DoOnHandle pattern)
- EmailTemplate handlers: Create, Update, Delete, Get, GetByService
- EmailChannel handlers: Create, Update, Delete, Get, GetByService

Ref: IT-628

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

HrynCo.NotificationService.Services/Core/RequestHandler.cs

@@ -0,0 +1,24 @@
+using HrynCo.NotificationService.DAL.Abstract;
+using HrynCo.NotificationService.Services.Logging;
+using MediatR;
+using Serilog;
+
+namespace HrynCo.NotificationService.Services.Core;
+
+public abstract class RequestHandler<TRequest, TResponse> : IRequestHandler<TRequest, TResponse>
+    where TRequest : IRequest<TResponse>
+{
+    protected RequestHandler(IContextualSerilogLogger<TRequest> logger, IUnitOfWork unitOfWork)
+    {
+        Logger = logger.Logger;
+        UnitOfWork = unitOfWork;
+    }
+
+    protected ILogger Logger { get; }
+    protected IUnitOfWork UnitOfWork { get; }
+
+    public Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken) =>
+        DoOnHandle(request, cancellationToken);
+
+    protected abstract Task<TResponse> DoOnHandle(TRequest request, CancellationToken cancellationToken);
+}

HrynCo.NotificationService.Services/Core/ServiceError.cs

@@ -0,0 +1,13 @@
+namespace HrynCo.NotificationService.Services.Core;
+
+public sealed record ServiceError
+{
+    public ServiceError(string message, ServiceErrorCode? code = null)
+    {
+        Message = message;
+        Code = code;
+    }
+
+    public ServiceErrorCode? Code { get; set; }
+    public string Message { get; set; }
+}

HrynCo.NotificationService.Services/Core/ServiceErrorCode.cs

@@ -0,0 +1,8 @@
+namespace HrynCo.NotificationService.Services.Core;
+
+public enum ServiceErrorCode
+{
+    NotFound = 1,
+    Conflict = 2,
+    InvalidRequest = 3,
+}

HrynCo.NotificationService.Services/Core/ServiceResult.cs

@@ -0,0 +1,17 @@
+namespace HrynCo.NotificationService.Services.Core;
+
+public record ServiceResult<TResult>
+{
+    public ServiceError? Error { get; private set; }
+    public bool IsSuccess { get; private set; }
+    public TResult? Result { get; private set; }
+
+    public static ServiceResult<TResult> Success(TResult result) =>
+        new() { IsSuccess = true, Result = result };
+
+    public static ServiceResult<TResult> Failure(ServiceError error) =>
+        new() { IsSuccess = false, Error = error };
+
+    public static ServiceResult<TResult> Failure(string message, ServiceErrorCode? code = null) =>
+        Failure(new ServiceError(message, code));
+}

HrynCo.NotificationService.Services/Core/ServiceResultHelper.cs

@@ -0,0 +1,9 @@
+namespace HrynCo.NotificationService.Services.Core;
+
+public static class ServiceResultHelper
+{
+    public static ServiceResult<T> Success<T>(T result) => ServiceResult<T>.Success(result);
+
+    public static ServiceResult<T> Failure<T>(string errorMessage, ServiceErrorCode? errorCode = null) =>
+        ServiceResult<T>.Failure(errorMessage, errorCode);
+}

HrynCo.NotificationService.Services/Core/Unit.cs

@@ -0,0 +1,6 @@
+namespace HrynCo.NotificationService.Services.Core;
+
+public readonly struct Unit
+{
+    public static readonly Unit Value = new();
+}