Most Matrix clients auto-set a room name when creating a DM (e.g.
"Alice & Bot" from participant display names), so the old
`is_direct and not has_explicit_name` heuristic classified virtually
all client-created DM rooms as "room", forcing require_mention gating
in legitimate one-on-one DMs.
member_count is now the primary DM signal: <=2 members means the room
is necessarily a 1:1 conversation, regardless of m.direct or an explicit
name. A room that grew to 3+ members but is still in stale m.direct is
still classified as a room (conflict flag set). Falls back to the
m.direct + name heuristic when the count is unavailable.
Also hardens _get_room_member_count with a joined_members API fallback
when the cache-backed state_store is empty.
Salvaged from #48554 by @justemu onto the current plugin adapter path
(gateway/platforms/matrix.py -> plugins/platforms/matrix/adapter.py).
Fixes#48551
4aa793345e