CacheMergedIT.shouldFetchMergedMessageValues fails only on GitHub Actions

[jfallows]

The CacheMergedIT.shouldFetchMergedMessageValues integration test has variable behavior as it merges multiple partition streams into a unified topic stream, where the order of messages in each partition is maintained but the total order of messages across all partitions can vary and still be correct.

We need to find a way to make the merge behavior more deterministic during this IT so that it can pass predictably in all environments and not be susceptible to any race conditions impacting the observed total message order.