Refactor solution structure and add RabbitMQ config

Reorganized the solution structure to align with a layered architecture:
- Replaced `src` folder with `core`, `infrastructure`, and `presentation`.
- Moved projects to their respective folders.
- Added `DigitalData.MessagingService.Publisher.Abstraction` project.
- Removed `DigitalData.MessagingService.Client` project.

Updated project configurations and nesting in the solution file.

Added `appsettings.Secrets.json` with RabbitMQ and email account settings:
- RabbitMQ configuration includes hostname, port, credentials, and queue/exchange details.
- Email configuration includes SMTP server details and credentials.

src/infrastructure/DigitalData.MessagingService.Infrastructure/Services/LimilabsEmailService.cs

@@ -0,0 +1,111 @@
+using System.Text;
+using DigitalData.MessagingService.Application.Common.Dtos;
+using DigitalData.MessagingService.Application.Common.Interfaces;
+using DigitalData.MessagingService.Domain.Exceptions;
+using Limilabs.Client.SMTP;
+using Limilabs.Mail;
+using Limilabs.Mail.Headers;
+using Microsoft.Extensions.Options;
+
+namespace DigitalData.MessagingService.Infrastructure.Services;
+
+/// <summary>
+/// Email service using Limilabs Mail.dll for SMTP operations (send-only).
+/// Commercial-grade library with superior Exchange support.
+/// SMTP configuration is injected via IOptions&lt;EmailAccountDto&gt; from appsettings.json.
+/// </summary>
+public class LimilabsEmailService(
+    IEncryptionService encryptionService,
+    IOptions<EmailAccountDto> smtpConfig) : IEmailService
+{
+    private readonly EmailAccountDto _smtpAccount = smtpConfig.Value;
+
+    // Register encoding provider for Limilabs (requires windows-1252 and other code pages)
+    static LimilabsEmailService()
+    {
+        Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
+    }
+
+    public async Task SendEmailAsync(string to, string subject, string body, bool isHtml = true, CancellationToken cancellationToken = default)
+    {
+        using var smtp = new Smtp();
+
+        try
+        {
+            await ConnectAndAuthenticateSmtpAsync(smtp);
+
+            var builder = new MailBuilder();
+            builder.From.Add(new MailBox(_smtpAccount.Username));
+            builder.To.Add(new MailBox(to));
+            builder.Subject = subject;
+
+            if (isHtml)
+            {
+                builder.Html = body;
+            }
+            else
+            {
+                builder.Text = body;
+            }
+
+            var mail = builder.Create();
+
+            var result = smtp.SendMessage(mail);
+
+            if (result.Status != SendMessageStatus.Success)
+            {
+                throw new InvalidOperationException($"Failed to send email. Status: {result.Status}");
+            }
+
+            smtp.Close();
+            await Task.CompletedTask; // For async consistency
+        }
+        catch (Limilabs.Client.ServerException ex)
+        {
+            DisconnectSafely(smtp);
+            throw new AuthenticationFailedException("SMTP authentication failed. Check credentials or OAuth2 configuration.", ex);
+        }
+        catch (Exception ex)
+        {
+            DisconnectSafely(smtp);
+            throw new InvalidOperationException("Failed to send email via SMTP server.", ex);
+        }
+    }
+
+    // --- Private Helper Methods ---
+
+    private async Task ConnectAndAuthenticateSmtpAsync(Smtp smtp)
+    {
+        if (_smtpAccount.SmtpUseSsl)
+        {
+            smtp.ConnectSSL(_smtpAccount.SmtpServer, _smtpAccount.SmtpPort);
+        }
+        else
+        {
+            smtp.Connect(_smtpAccount.SmtpServer, _smtpAccount.SmtpPort);
+        }
+
+        if (_smtpAccount.UseOAuth2)
+        {
+            throw new NotSupportedException("OAuth2 is not configured for this SMTP account. UseOAuth2 must be false.");
+        }
+        else
+        {
+            var password = _smtpAccount.PasswordEncrypted ? encryptionService.Decrypt(_smtpAccount.Password) : _smtpAccount.Password;
+
+            smtp.Login(_smtpAccount.Username, password);
+        }
+
+        await Task.CompletedTask; // For async consistency
+    }
+
+    private static void DisconnectSafely(Smtp smtp)
+    {
+        try
+        {
+            if (smtp.Connected)
+                smtp.Close();
+        }
+        catch { /* Ignore disconnect errors */ }
+    }
+}