feat(rabbitmq): pool RabbitMQ channels in RabbitMqMessageTransport

[samtrion]

Summary

• Add IRabbitMqChannelPool / RabbitMqChannelPool in NetEvolve.Pulse.Internals, backed by a ConcurrentQueue<IRabbitMqChannelAdapter> of idle channels and a SemaphoreSlim capped at MaxChannelPoolSize concurrent rentals. RentAsync dequeues an open idle channel, disposing and skipping any closed one, or creates a new channel via IRabbitMqConnectionAdapter when none are available; Return re-queues an open channel or disposes a closed one, always releasing the rental slot exactly once (including when the pool itself has been disposed while the channel was rented out).
• Add RabbitMqTransportOptions.MaxChannelPoolSize (default 10).
• Rework RabbitMqMessageTransport to rent a channel from the pool per SendAsync call and to rent a single channel for the whole SendBatchAsync batch (published sequentially on it), always returning the channel in a finally block. This removes the old single shared _channel field, _initializationLock, and _publishLock — per-channel publish safety is now inherent because each rental is exclusive. IsHealthyAsync now delegates to the pool's own health check.
• Register IRabbitMqChannelPool → RabbitMqChannelPool as a singleton via TryAddSingleton in UseRabbitMqTransport, so calling it more than once does not duplicate the registration.
• Update unit tests to the new pool-based design (constructor now takes IRabbitMqChannelPool instead of IRabbitMqConnectionAdapter) and add RabbitMqChannelPoolTests covering: concurrent rent capped at MaxChannelPoolSize, returning an open channel makes it available for reuse, returning a closed channel disposes it and the next rent creates a fresh one, pool exhaustion causing a rent to wait until a return happens, and dispose (idempotent, disposes all idle channels, Return/RentAsync behave safely after dispose).
• Update the RabbitMQ integration test fixture and one behavior test (IsHealthyAsync before any channel exists now correctly reports healthy based on the connection/pool, not on the previous single-field-existence quirk).

Closes #241

Deviations from the issue text

• The issue's out-of-the-box constructor code snippets were adapted to match the actual codebase (existing IRabbitMqChannelAdapter / IRabbitMqConnectionAdapter types were reused as-is, matching the codebase context already given).
• SendBatchAsync rents one channel for the entire batch and publishes its messages sequentially on it, rather than renting a channel per message. This was called out as an explicit choice in the issue ("pick whichever keeps the implementation simplest and correct"); renting once per batch avoids rent/return churn for what is inherently a single-threaded, ordered sequence of publishes.
• RabbitMqMessageTransport's constructor now takes IRabbitMqChannelPool instead of IRabbitMqConnectionAdapter, since the channel pool fully owns channel creation/health against the connection. This is a necessary consequence of removing the transport's private channel-management, and is reflected in the DI registration and the updated unit tests.
• IsHealthyAsync no longer requires that a channel has already been created before reporting healthy (the old code returned false until the first SendAsync/SendBatchAsync call because a _channel field had not been set yet). With pooling there is no single persistent "the channel" to check, so health now reflects the pool/connection state directly. One integration test (IsHealthyAsync_Before_first_send_returns_false) was updated to IsHealthyAsync_Before_first_send_returns_true_when_connection_open to reflect this intentional, documented behavior change.

Test plan

• dotnet build Pulse.slnx — builds clean (net8.0/net9.0/net10.0)
• dotnet test Tests/NetEvolve.Pulse.Tests.Unit --treenode-filter "/*/*/*RabbitMq*/*" — 156 passed, 0 failed, 0 skipped (net8.0/net9.0/net10.0)
• csharpier format . run before committing
• Integration tests (NetEvolve.Pulse.Tests.Integration) require Docker/Testcontainers and were not run in this environment, but the project builds successfully with the updated pool-based CreateTransport helper.

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[codecov]

Codecov Report

❌ Patch coverage is 98.83721% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.05%. Comparing base (c82da38) to head (703b098).

Files with missing lines	Patch %	Lines
...ve.Pulse.RabbitMQ/Internals/RabbitMqChannelPool.cs	92.85%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #675      +/-   ##
==========================================
- Coverage   96.07%   96.05%   -0.02%     
==========================================
  Files         172      188      +16     
  Lines        7179     7373     +194     
  Branches      661      679      +18     
==========================================
+ Hits         6897     7082     +185     
- Misses        136      145       +9     
  Partials      146      146              

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