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>
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