WebTransport tester and echo server status

// Example: echo one message over a bidirectional stream.
const transport = new WebTransport('https://example.com:4433/echo');
await transport.ready;

const stream = await transport.createBidirectionalStream();
const writer = stream.writable.getWriter();
await writer.write(new TextEncoder().encode('hello'));
await writer.close();

const reader = stream.readable.getReader();
const decoder = new TextDecoder();
for (;;) {
  const { value, done } = await reader.read();
  if (done) break;
  console.log('echo:', decoder.decode(value, { stream: true }));
}
transport.close();

// Example: one datagram. Datagrams are unreliable: no retransmission,
// so a reply may never arrive.
const transport = new WebTransport('https://example.com:4433/echo');
await transport.ready;

const dg = transport.datagrams;
const writer = (dg.createWritable?.() ?? dg.writable).getWriter();
await writer.write(new TextEncoder().encode('ping'));

const reader = dg.readable.getReader();
const { value } = await reader.read();
console.log('echo:', new TextDecoder().decode(value));