[SPARK-50360][SS][FOLLOWUP][TESTS] Changelog reader test improvements

[WweiL]

What changes were proposed in this pull request?

In some test cases added by #48880, changelogReader.closeIfNeeded() is called before assert(entries.size == expectedEntries.size), while entries are defined as entries = changelogReader.toSeq, which triggers the read.

So the logic is

entries = changelogReader.toSeq
changelogReader.closeIfNeeded()
assert(entries.size == expectedEntries.size)

This is fine in most cases, but we find out that in certain environments, the toSeq is lazily evaluated; therefore, not until the assert, the actual read won't be triggered. To be extra safe, change the order of the above logic to be the following:

entries = changelogReader.toSeq
assert(entries.size == expectedEntries.size)
changelogReader.closeIfNeeded()

Why are the changes needed?

Test case improvement

Does this PR introduce any user-facing change?

No

How was this patch tested?

Test only change

Was this patch authored or co-authored using generative AI tooling?

No

[HeartSaVioR]

+1

[HeartSaVioR]

Thanks! Merging to master.