Previously _check_inbound_policy returned (False, chat_id) for DMs
that failed the allowlist and the caller dropped them — so unapproved
DM senders never saw a pairing code. Mirror Slack: when the policy
gate denies a DM but dm.enabled is true, still call
_handle_message(content="", is_dm=True) so BaseChannel can issue the
pairing reply. Group denials stay a hard drop.
Combined with the previous is_dm forwarding, unapproved DM senders
now receive a pairing code through the standard flow.
Addresses review comment on PR #3852.