Previously, unmapped aggs try to delegate reduction to a sibling agg that is
mapped. That delegated agg will run the reductions, and also
reduce any pipeline aggs. But because delegation comes before running
pipelines, the unmapped agg _also_ tries to run pipeline aggs.
This causes the pipeline to run twice, and potentially double it's output
in buckets which can create invalid JSON (e.g. same key multiple times)
and break when converting to maps.
This fixes by sorting the list of aggregations ahead of time so that mapped
aggs appear first, meaning they preferentially lead the reduction. If all aggs
are unmapped, the first unmapped agg simply creates a new unmapped object
and returns that for the reduction.
This means that unmapped aggs no longer defer and there is no chance for
a secondary execution of pipelines (or other side effects caused by deferring
execution).
Closes#33514