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/DependencyInjection.cs

@@ -33,7 +33,7 @@ public static class DependencyInjection
         services.AddSingleton<IEncryptionService, DataProtectionEncryptionService>();
 
         // --- Email Queue (RabbitMQ) ---
-        services.AddSingleton<SendingEmailConsumer>();
+        services.AddSingleton<SendingEmailConsumerPool>();
         services.AddMessagingServicePublisher();
 
         // --- RabbitMQ Configuration ---