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/Queue/SendingEmailConsumerPool.cs

@@ -0,0 +1,51 @@
+using DigitalData.MessagingService.Application.Common.Interfaces;
+using DigitalData.MessagingService.RabbitMQ;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+
+namespace DigitalData.MessagingService.Infrastructure.Queue;
+
+/// <summary>
+/// Manages a pool of <see cref="SendingEmailConsumer"/> instances that compete for messages
+/// on the same RabbitMQ queue (competing consumers pattern).
+/// Each consumer owns a dedicated channel, so they process messages fully in parallel
+/// without any shared locking or synchronization primitives.
+/// </summary>
+public sealed class SendingEmailConsumerPool : IAsyncDisposable
+{
+    private readonly List<SendingEmailConsumer> _consumers;
+    private readonly ILogger<SendingEmailConsumerPool>? _logger;
+    private readonly int _concurrency;
+
+    public SendingEmailConsumerPool(
+        IOptions<RabbitMqConfiguration> config,
+        IEmailService emailService,
+        RabbitMqConnectionFactory cnnFactory,
+        ILogger<SendingEmailConsumerPool>? logger = null,
+        ILogger<SendingEmailConsumer>? consumerLogger = null)
+    {
+        _logger = logger;
+        _concurrency = config.Value.ConsumerConcurrency;
+
+        _consumers = [.. Enumerable
+            .Range(0, _concurrency)
+            .Select(_ => new SendingEmailConsumer(config.Value.QueueName, emailService, cnnFactory, consumerLogger))];
+    }
+
+    /// <summary>
+    /// Starts all consumers in parallel. Each consumer opens its own channel and begins listening.
+    /// </summary>
+    public async Task InitAsync()
+    {
+        _logger?.LogInformation("Starting {Count} competing email consumers.", _concurrency);
+
+        await Task.WhenAll(_consumers.Select(c => c.InitAsync()));
+
+        _logger?.LogInformation("All {Count} email consumers started.", _concurrency);
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        await Task.WhenAll(_consumers.Select(async c => await c.DisposeAsync().AsTask()));
+    }
+}