Fix flaky test: test pubsub channels and shardchannels separation #4927

[hank95179]

Description

This PR fixes the flaky test test pubsub channels and shardchannels separation_true tracked in #4927.

The failure is caused by the eventual consistency model of the Redis Cluster Gossip protocol. When a client subscribes to a channel on one node, it takes a small amount of time for this metadata to propagate to other nodes. The test was asserting the existence of the channel immediately after subscription, leading to intermittent failures when the target node hadn't received the update yet.

Changes

• Implemented a polling mechanism to wait for the propagation to complete.
• The test now retries the pubsubChannels check (up to a timeout) until the expected channel appears, ensuring the cluster metadata is synchronized.

Verification

• Verified locally that the test passes consistently with the new polling logic applied.

Related Issue

Fixes #4927