Fix semaphore leak in RabbitMqChannelAgent.EnsureInitiated()

[smatushonok]

Summary

Fix a deadlock caused by a semaphore not being released in EnsureInitiated() when the channel was already initialized by a concurrent call.

Problem

When multiple concurrent PublishAsync calls hit a cold (uninitialized) sender:

1. All calls pass the first Channel is not null check
2. First call acquires the semaphore, initializes the channel, releases semaphore
3. Second call acquires the semaphore, sees channel is now initialized, returns without releasing the semaphore
4. Remaining calls wait forever on the semaphore

Reproduction

await app.StartAsync();
var bus = scope.ServiceProvider.GetRequiredService<IMessageBus>();

// This deadlocks - only ~2 of 5 messages complete
var tasks = Enumerable.Range(1, 5)
    .Select(id => bus.PublishAsync(id).AsTask())
    .ToArray();
await Task.WhenAll(tasks);