Fix consumer count for quorum queues in queue.declare

[kjnilsson]

Previously, rabbit_fifo_client:stat/2 incorrectly returned the number of checked out messages (num_checked_out) instead of the actual number of consumers (num_consumers). This caused queue.declare operations on quorum queues to return the unacknowledged message count in the consumer_count field rather than the true number of active consumers.

This commit:

• Updates rabbit_fifo_client:stat/2 to correctly extract num_consumers from the ra machine overview.
• Adds queue_declare_passive_consumer_count to quorum_queue_SUITE to verify that passive declares return the correct consumer count when subscribing and cancelling consumers.

[Gsantomaggio]

It works correctly now, tested with:

 Environment environment = new AmqpEnvironmentBuilder()
                .build();
        Connection connection = environment.connectionBuilder().build();
        connection.management().queue("my_qq").quorum().queue().declare();

        for (int i = 0; i < 15; i++) {
            Consumer consumer = connection.consumerBuilder().queue("my_qq").messageHandler((context, message) -> {
                System.out.println("Received message: " + new String(message.body()));
            }).build();
        }
       Management.QueueInfo info = connection.management().queueInfo("my_qq");

       System.out.println("Consumer count " + info.consumerCount() );

out:

Consumer count 15