Checking maxRowsPerSegment after segment merge in dynamic partitioning

### Description

In this issue, the segment size refers to the row count in the segment. In dynamic partitioning, the task compares `maxRowsPerSegment` and the current `Sink` size to determine whether the segment hits the max segment size limit and needs to be pushed to deep storage. Here, the `Sink` size is the sum of in-memory segment size and the size of all persisted segments _before_ they are merged. As a result, when the rollup is on, the `Sink` size can be larger than the actual segment size. This can create smaller segments than `maxRowsPerSegment` unnecessarily.

Start by locating the dynamic partitioning logic that compares maxRowsPerSegment with the Sink size, then trace how rollup changes the size after persisted segments are merged. Add or update tests covering the post-merge segment row count; done means segments are not pushed unnecessarily early when rollup is enabled.

Written by the indexing model from the issue text.