fix Subquery annotation detection in dnfs() for Django 6.0+

[audriusm]

Problem

Django 6.0 changed how Subquery expressions are stored in qs.query.annotations. In Django 5.x, annotation values remain as Subquery instances. In Django 6.0+, they are resolved into raw django.db.models.sql.query.Query objects during .annotate().

The annotation dependency detection in tree.dnfs() only checks for Subquery instances:

subqueries = (query_dnf(getattr(q, 'query', None))
              for q in qs.query.annotations.values() if isinstance(q, Subquery))

On Django 6, isinstance(q, Subquery) returns False for the resolved Query objects, so cross-table Subquery dependencies are completely invisible to cache invalidation.

This means any queryset with Subquery annotations on a cacheops-enabled model won't invalidate its cache when the subqueried table is modified - the cached result stays stale until timeout.

How I found this

Came up while upgrading a project from Django 5.2 to 6.0. An endpoint annotating a cached model with a Subquery on a different table was returning stale data after writes to the subqueried table. Inspecting dnfs() output confirmed it only reported the primary table - the subqueried table was missing entirely.

Fix

Check for both Subquery instances (Django <6) and resolved Query objects (Django 6+) when scanning annotations for table dependencies.

The existing test_365 covers this scenario and would fail on Django 6 without this fix.

Added test_366 test to verify this fix.

[Suor]

Merged, thanks