Cherry-picked from PR #2575 by ticketclosed-wontfix.
Filters out Discord system messages (thread renames, pins, member joins,
boosts) that were being treated as regular user messages.
Follow-up fix: also allow MessageType.reply (value 19) — the original
filter only allowed MessageType.default, which would silently drop all
reply-based interactions.
Added pytest.importorskip for discord dependency in tests.