feat: RabbitMQ channel pooling in RabbitMqMessageTransport

[samtrion]

User Story

As a developer using RabbitMQ with Pulse under high throughput, I want RabbitMqMessageTransport to use a channel pool instead of a single shared channel, so that concurrent publish operations are not serialized and throughput scales with connection parallelism.

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Background

RabbitMqMessageTransport is registered as Singleton and lazily initializes a single IRabbitMqChannelAdapter. All concurrent SendAsync and SendBatchAsync calls share this one channel, serializing all publishes. Under batch-send scenarios or high concurrency this becomes a bottleneck. RabbitMQ channels are lightweight; pooling them allows concurrent publishes over the same connection without multiplying connections.

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Requirements

• Define IRabbitMqChannelPool interface:

  ValueTask<IRabbitMqChannelAdapter> RentAsync(CancellationToken cancellationToken);
  void Return(IRabbitMqChannelAdapter channel);
  Task<bool> IsHealthyAsync(CancellationToken cancellationToken);

• Add int MaxChannelPoolSize { get; set; } = 10; to RabbitMqTransportOptions.
• Implement RabbitMqChannelPool : IRabbitMqChannelPool, IDisposable:
  • ConcurrentQueue<IRabbitMqChannelAdapter> for pooled channels.
  • SemaphoreSlim(MaxChannelPoolSize) to limit concurrent checkouts.
  • RentAsync: acquire semaphore, dequeue existing channel or create new; if dequeued channel is closed, dispose it and create a new one.
  • Return: if channel is open, enqueue back; if closed, dispose and release semaphore.
  • Dispose: dispose all pooled channels.
• Update RabbitMqMessageTransport to replace _channel / _initializationLock with IRabbitMqChannelPool; use pool.RentAsync / pool.Return in SendAsync and SendBatchAsync; always Return in a finally block.
• Register IRabbitMqChannelPool → RabbitMqChannelPool as Singleton in RabbitMqMediatorBuilderExtensions.

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Acceptance Criteria

• RabbitMqChannelPool correctly limits concurrent checkouts to MaxChannelPoolSize.
• A closed channel returned to the pool is disposed; a new one is created on next RentAsync.
• Return is always called after SendAsync, even when an exception occurs.
• RabbitMqMessageTransport no longer holds a single channel field.
• IRabbitMqChannelPool is registered as Singleton.
• Unit tests cover: concurrent rent, return of open channel, return of closed channel, pool exhaustion waits, dispose.

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Out of Scope

• Publisher confirms.
• Connection-level pooling.