Regression was introduced by fff5186 but was due a long standing issue.
AggregateQuery was abusing Query.subquery: bool by stashing its
compiled inner query's SQL for later use in its compiler which made
select_format checks for Query.subquery wrongly assume the provide
query was a subquery.
This patch prevents that from happening by using a dedicated
inner_query attribute which is compiled at a later time by
SQLAggregateCompiler.
Moving the inner query's compilation to SQLAggregateCompiler.compile
had the side effect of addressing a long standing issue with
aggregation subquery pushdown which prevented converters from being
run. This is now fixed as the aggregation_regress adjustments
demonstrate.
Refs #25367.
Thanks Eran Keydar for the report.
c2d4926702