E2E test: Multiple consumers reading from the same stream concurrently

[s2x]

Closes #174

Problem

No E2E test verifies that multiple consumers (on separate connections) can read from the same stream independently. Each consumer should receive all messages and maintain independent offsets.

Expected test

public function testMultipleConsumersOnSameStream(): void
{
    $stream = 'test-multi-consumer-' . uniqid();
    $conn1 = Connection::create(...);
    $conn2 = Connection::create(...);

    $conn1->createStream($stream);

    // Publish messages
    $producer = $conn1->createProducer($stream);
    $producer->sendBatch([...5 AMQP messages...]);
    $producer->waitForConfirms();
    $producer->close();

    // Consumer 1 on conn1
    $consumer1 = $conn1->createConsumer($stream, OffsetSpec::first(), name: 'consumer-1');
    // Consumer 2 on conn2
    $consumer2 = $conn2->createConsumer($stream, OffsetSpec::first(), name: 'consumer-2');

    // Both should receive all 5 messages
    $msgs1 = $this->readAll($consumer1, 5);
    $msgs2 = $this->readAll($consumer2, 5);

    $this->assertCount(5, $msgs1);
    $this->assertCount(5, $msgs2);

    // Store different offsets
    $consumer1->storeOffset($msgs1[2]->getOffset()); // offset at message 3
    $consumer2->storeOffset($msgs2[4]->getOffset()); // offset at message 5

    // Verify independent offsets
    $offset1 = $conn1->queryOffset('consumer-1', $stream);
    $offset2 = $conn2->queryOffset('consumer-2', $stream);
    $this->assertNotSame($offset1, $offset2);
}

Why it matters

• Multiple consumers is the standard pattern for RabbitMQ Streams (fan-out)
• Each consumer must have independent offset tracking
• Tests that consumers on different connections don't interfere with each other

Acceptance criteria

• Two consumers on different connections receive all messages independently
• Each consumer can store and query its own offset
• Offsets are independent (storing one doesn't affect the other)