Consider row-count skew when skipping aggregate repartitioning #27268

Skipping aggregate repartitioning can preserve a heavily skewed input layout, leaving most of the grouping work on one worker even when more execution partitions are available.

In Jay's benchmark on #27135, one of four files held 97% of 16M rows. With preserve_file_partitions enabled and mostly unique groups, the SinglePartitioned plan took 4.11 times as long as the shuffle plan at target_partitions=8. The two-argument date_bin showed the same behavior.

Consider per-partition row counts when deciding whether to skip aggregate repartitioning, so a shuffle can still be used when its load-balancing benefit outweighs its cost. Related to #19269, which describes the same trade-off for hash-subset partitioning.