(fix): keep echo backend alive for multi-send test, fix PHPStan type

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: abnegate

tests/Integration/TCPServerTest.php

@@ -40,8 +40,11 @@ public function testTCPAdapterGetConnectionAndClose(): void
 
             Coroutine::create(function () use ($backend): void {
                 $backend->handle(function (Coroutine\Server\Connection $connection): void {
-                    $data = $connection->recv();
-                    if ($data !== '' && $data !== false) {
+                    while (true) {
+                        $data = $connection->recv();
+                        if (! \is_string($data) || $data === '') {
+                            break;
+                        }
                         $connection->send($data);
                     }
                     $connection->close();