test(node-integration-tests): Wait for kafkajs GROUP_JOIN before producing (#21074)

## Summary

- Replace the fixed 4s `setTimeout` in
`suites/tracing/kafkajs/scenario.mjs` with a deterministic wait on the
kafkajs `GROUP_JOIN` event.
- Also `await consumer.run(...)` instead of leaving the promise
dangling.

## Why

The kafkajs integration test flaked with the consumer transaction never
arriving within the test timeout. The scenario was:

1. `consumer.subscribe(...)` (awaited)
2. `consumer.run(...)` — **not** awaited
3. `await new Promise(resolve => setTimeout(resolve, 4000))`
4. `producer.send(...)`

On slow CI runners, the consumer group rebalance / join can take longer
than the fixed 4 seconds. Although `fromBeginning: true` lets a
late-joining consumer still read offset 0, when the join is slow enough
the consumer transaction isn't created before the per-test timeout.

Listening for the `GROUP_JOIN` event removes the timing assumption
entirely: we proceed exactly when the consumer is in its group and
actively polling. The listener is registered before `consumer.run()` so
it cannot miss the event.

This mirrors the prior fix for the same suite (#20189, which addressed
transaction ordering) and the same race-on-startup pattern already
documented in the amqplib scenario.

Fixes #21044

Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: github-actions[bot]

dev-packages/node-integration-tests/suites/tracing/kafkajs/scenario.mjs

@@ -23,15 +23,23 @@ async function run() {
   await consumer.connect();
   await consumer.subscribe({ topic: 'test-topic', fromBeginning: true });
 
-  consumer.run({
+  // Resolve once the consumer has actually joined its group. A fixed sleep
+  // here was racy on slow CI runners: if the producer sent before the
+  // consumer joined, the consumer transaction sometimes wasn't created
+  // within the test timeout. Register the listener before `run()` so the
+  // event can't fire before we're listening.
+  const groupJoined = new Promise(resolve => {
+    consumer.on(consumer.events.GROUP_JOIN, () => resolve());
+  });
+
+  await consumer.run({
     eachMessage: async ({ message }) => {
       // eslint-disable-next-line no-console
       console.debug('Received message', message.value.toString());
     },
   });
 
-  // Wait for the consumer to be ready
-  await new Promise(resolve => setTimeout(resolve, 4000));
+  await groupJoined;
 
   await producer.send({
     topic: 'test-topic',