E2E test: Server-initiated close handling

[s2x]

Closes #151

Problem

StreamConnection::dispatchServerPush() handles server-initiated close (key 0x0016) by sending a CloseResponse and closing the connection. However, no E2E test verifies this behavior. This is a critical path — in production, the server may close connections due to policy violations, resource limits, or administrative actions.

How to trigger

Server-initiated close can be triggered by:

1. Deleting a stream while a consumer is subscribed to it (may trigger MetadataUpdate + Close)
2. Using the RabbitMQ management API to force-close a connection
3. Server shutdown/restart

Expected test

public function testServerInitiatedCloseIsHandledGracefully(): void
{
    $connection = Connection::create(...);
    $streamName = 'test-server-close-' . uniqid();
    $connection->createStream($streamName);

    // Force close via management API
    // curl -X DELETE http://localhost:15672/api/connections/...

    // Next operation should detect the closed connection
    $this->expectException(\RuntimeException::class);
    $this->expectExceptionMessage('Connection closed');
    $connection->createStream('another-stream');
}

Why it matters

• Critical production scenario — servers close connections regularly
• The dispatchServerPush() Close handler sends a response and calls $this->close(), but this path is untested
• Without this test, regressions in close handling could cause silent data loss

Acceptance criteria

• Verify isConnected() returns false after server-initiated close
• Verify subsequent operations throw meaningful exceptions
• Verify the client sends a proper CloseResponse before disconnecting