[router] Introduce router integration tests (#10086)

sgl-router/py_test/fixtures/ports.py

@@ -0,0 +1,8 @@
+import socket
+
+
+def find_free_port() -> int:
+    """Return an available TCP port on localhost."""
+    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
+        s.bind(("127.0.0.1", 0))
+        return s.getsockname()[1]