fix: resolve 5 PR review blockers for comments v2

1. Reply HTMX target: server sends HX-Retarget/HX-Reswap headers to
   insert replies inside parent comment's .replies-container div
2. Empty thread swap target: always render #comments-list container
   even when no approved comments exist
3. Reaction hydration: add _annotate_reaction_counts() helper that
   hydrates reaction_counts and user_reacted on comments in
   get_context(), comment_poll(), and single-comment responses
4. HTMX error swap: return 200 instead of 422 for form errors since
   HTMX 2 doesn't swap 4xx responses by default
5. Vary header: use patch_vary_headers() instead of direct assignment
   to avoid overwriting existing Vary directives

Also fixes _get_session_key() to handle missing session attribute
(e.g. from RequestFactory in performance tests).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

apps/blog/models.py

@@ -306,11 +306,16 @@ class ArticlePage(SeoMixin, Page):
         from django.conf import settings
 
         from apps.comments.models import Comment
+        from apps.comments.views import _annotate_reaction_counts, _get_session_key
 
         approved_replies = Comment.objects.filter(is_approved=True).select_related("parent")
-        ctx["approved_comments"] = self.comments.filter(is_approved=True, parent__isnull=True).prefetch_related(
-            Prefetch("replies", queryset=approved_replies)
+        comments = list(
+            self.comments.filter(is_approved=True, parent__isnull=True).prefetch_related(
+                Prefetch("replies", queryset=approved_replies)
+            )
         )
+        _annotate_reaction_counts(comments, _get_session_key(request))
+        ctx["approved_comments"] = comments
         ctx["turnstile_site_key"] = getattr(settings, "TURNSTILE_SITE_KEY", "")
         return ctx