SingleBucket aggs need to reduce their bucket's pipelines first #50103
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When decoupling the pipeline reduction from regular agg reduction, MultiBucket aggs were modified to reduce their bucket's pipeline aggs first before reducing the sibling aggs. This modification was missed on SingleBucket aggs, meaning any SingleBucket would fail to reduce any pipeline sub-aggs
The fix is straightforward: single bucket aggs should implement a similar override and reduce the aggs in their bucket first. This commit implements that behavior, and adds a test (using nested as an example).
Closes #50054