[pkuzmickas]

Summary

Restore the source split logs in the Flink DataStream runner that #39874 removed when it moved splitting into FlinkSourceSplitUtils.

Before #39874, the static and lazy enumerators logged:

• Split bounded source {} in {} splits
• Split source {} to {} splits (unbounded)

After the refactor, only the opt-in size-based enumerator logged split counts. The default lazy and static paths logged nothing about splitting.

Why

These logs are the main way to diagnose source skew in batch jobs. For example, comparing the split count with the source parallelism shows whether a source produces too few splits to distribute work evenly.

Changes

• Log in FlinkSourceSplitUtils.splitBoundedSource / splitUnboundedSource, so every enumerator emits the log.
• Keep the original message prefixes so existing log searches still match. The bounded log also includes the estimated source size and desired split size.
• Remove the size-based enumerator's post-split log, which would now be a duplicate. Its existing assignment-mode log still reports the selected mode.

---

• Logging-only change; no CHANGES.md entry needed.

[pkuzmickas]

Hey @Abacn could you please take a look? 🙏 I accidentally removed this in the previous PR we merged.

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