[fix](compaction) Avoid repeatedly compacting large cumulative rowset

[Yukang-Lian]

Proposed changes

• Remove the promotion-size early return in size-based cumulative compaction rowset selection so level-size trimming can exclude an oversized leading rowset.
• Keep the level-size trimming loop from trimming a single candidate rowset to an empty input set.
• Apply the same behavior to both cloud and storage cumulative compaction policies.
• Add regression coverage for the large-head/small-tail case and the single-rowset guard in both paths.

[hello-stephen]

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