The message path drops a message from an ignored channel (_prefilter_inbound)
and, when allowed_channels is set, from any non-1:1 conversation outside it
(_channel_gate_allows). _handle_slash_command applied neither, so a slash
command ran in a channel the operator had excluded. Apply both gates by
channel id alone, before any lookup, so a failed conversations.info can never
drop a control command.
Rebuilt from 2b7cde506c (#128767). Fixes#130358.
b376591778