feat: implement SendBatchAsync and integration tests for NetEvolve.Pulse.RabbitMQ

[Copilot]

The RabbitMQ transport (NetEvolve.Pulse.RabbitMQ) was missing a SendBatchAsync override, leaving it reliant on the default Parallel.ForEachAsync implementation — which causes concurrent access on a shared IChannel that is not thread-safe. Integration tests against a real broker were also absent.

Transport changes

• RabbitMqMessageTransport: added SendBatchAsync override that acquires the channel once and publishes all messages sequentially on that single channel
• Extracted private PublishAsync helper shared by both SendAsync and SendBatchAsync to eliminate duplication

services.AddPulse(c => c.UseRabbitMqTransport(options =>
{
    options.ExchangeName = "events";
}));

// SendBatchAsync now uses one channel, publishes sequentially — safe and efficient
await transport.SendBatchAsync(outboxMessages, ct);

Unit tests (NetEvolve.Pulse.Tests.Unit)

Added five tests for SendBatchAsync covering:

• Null-argument guard
• All messages published with correct properties
• Single channel reused across the entire batch
• Empty collection produces no publish calls
• Channel reuse when mixed with prior SendAsync calls

Integration tests (NetEvolve.Pulse.Tests.Integration)

• RabbitMqContainerFixture: manages a Testcontainers.RabbitMq container lifecycle per test session
• RabbitMqMessageTransportIntegrationTests: verifies SendAsync, SendBatchAsync, and IsHealthyAsync (before first send → false; after first send with open connection → true) against a live broker