Stabilize flaky PUBSUB multi-client test by waiting for subscriptions

[hank95179]

Stabilize flaky PUBSUB test by waiting for subscriptions to establish

Issue link

This Pull Request is linked to issue (URL): #4695

Similar fix for reference: #4623

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What & Why

Flaky test: PubSub › pubsub combined exact and pattern multiple clients test_true_0

Root cause
Right after creating clients with PUBSUB subscriptions, the test immediately publishes messages. On slower or contended environments, publishes can race ahead of the subscription handshake (server hasn’t fully registered the subscriber yet), causing dropped messages and timeouts.

Fix (minimal, no hooks, no new helpers):
Insert a short await after subscription setup and before publishing. This gives the subscription time to become active.

// Wait for subscriptions to be established (cluster & standalone)
await new Promise((resolve) => setTimeout(resolve, 1000));

[jamesx-improving]

LGTM

[jamesx-improving]

Hi @hank95179 , thank you for your contribution! The fix looks good, the only thing is, we have a policy that all commits needs to have signing and signoff. We can help merge your PR after these are being taken care of.

Thanks again!