Introduce RabbitMQ consumer pool for parallel processing

Enhanced RabbitMQ email processing by introducing a `SendingEmailConsumerPool` to enable the competing consumers pattern. Each consumer operates on its own channel, improving scalability and thread safety.

- Added `SendingEmailConsumerPool` to manage multiple consumers.
- Updated `DependencyInjection` to register the consumer pool.
- Refactored `SendingEmailConsumer` for better logging and error handling.
- Updated `AsyncInitWorker` to initialize the consumer pool.
- Added `ConsumerConcurrency` to RabbitMQ configuration.
- Improved error handling in `LimilabsEmailService` with detailed SMTP error messages.

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

@@ -28,7 +28,7 @@ public class LimilabsEmailService(
     public async Task SendEmailAsync(EmailContext context, CancellationToken cancellationToken = default)
     {
         using var smtp = new Smtp();
-
+        ISendMessageResult? result = null;
         try
         {
             await ConnectAndAuthenticateSmtpAsync(smtp, context.Sender);
@@ -48,25 +48,24 @@ public class LimilabsEmailService(
 
             var mail = builder.Create();
 
-            var result = await smtp.SendMessageAsync(mail, cancellationToken);
+            result = await smtp.SendMessageAsync(mail, cancellationToken);
 
             if (result.Status != SendMessageStatus.Success)
             {
-                throw new InvalidOperationException($"Failed to send email. Status: {result.Status}");
+                throw new InvalidOperationException($"Failed to send email. Status: {result.Status}. {ErrorMessageBuilder(result)}");
             }
 
             await smtp.CloseAsync(cancellationToken);
-            await Task.CompletedTask; // For async consistency
         }
         catch (Limilabs.Client.ServerException ex)
         {
             await smtp.CloseSafelyAsync();
-            throw new AuthenticationFailedException("SMTP authentication failed. Check credentials or OAuth2 configuration.", ex);
+            throw new AuthenticationFailedException($"SMTP authentication failed. Check credentials or OAuth2 configuration. {ErrorMessageBuilder(result)}", ex);
         }
         catch (Exception ex)
         {
             await smtp.CloseSafelyAsync();
-            throw new InvalidOperationException("Failed to send email via SMTP server.", ex);
+            throw new InvalidOperationException($"Failed to send email via SMTP server. {ErrorMessageBuilder(result)}", ex);
         }
     }
 
@@ -92,4 +91,21 @@ public class LimilabsEmailService(
             await smtp.LoginAsync(smtpAccount.Username, password);
         }
     }
-}
+
+    private static string ErrorMessageBuilder(ISendMessageResult? result = null)
+    {
+        if(result is null || result.GeneralErrors.Count == 0)
+            return string.Empty;
+        else if(result.GeneralErrors.Count == 1)
+            return $"Error: {result.GeneralErrors.FirstOrDefault()}";
+
+        var message = new StringBuilder("Errors:\n");
+
+        foreach (var error in result.GeneralErrors)
+        {
+            message.AppendLine($"   • {error}");
+        }
+
+        return message.ToString();
+    }
+}