Fixed #31910 -- Fixed crash of GIS aggregations over subqueries.

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.

django/db/models/sql/query.py

@@ -17,9 +17,7 @@ from collections.abc import Iterator, Mapping
 from itertools import chain, count, product
 from string import ascii_uppercase
 
-from django.core.exceptions import (
-    EmptyResultSet, FieldDoesNotExist, FieldError,
-)
+from django.core.exceptions import FieldDoesNotExist, FieldError
 from django.db import DEFAULT_DB_ALIAS, NotSupportedError, connections
 from django.db.models.aggregates import Count
 from django.db.models.constants import LOOKUP_SEP
@@ -449,8 +447,9 @@ class Query(BaseExpression):
         if (isinstance(self.group_by, tuple) or self.is_sliced or existing_annotations or
                 self.distinct or self.combinator):
             from django.db.models.sql.subqueries import AggregateQuery
-            outer_query = AggregateQuery(self.model)
             inner_query = self.clone()
+            inner_query.subquery = True
+            outer_query = AggregateQuery(self.model, inner_query)
             inner_query.select_for_update = False
             inner_query.select_related = False
             inner_query.set_annotation_mask(self.annotation_select)
@@ -492,13 +491,6 @@ class Query(BaseExpression):
                 # field selected in the inner query, yet we must use a subquery.
                 # So, make sure at least one field is selected.
                 inner_query.select = (self.model._meta.pk.get_col(inner_query.get_initial_alias()),)
-            try:
-                outer_query.add_subquery(inner_query, using)
-            except EmptyResultSet:
-                return {
-                    alias: None
-                    for alias in outer_query.annotation_select
-                }
         else:
             outer_query = self
             self.select = ()