Lift the 4 inbound-message authorization methods out of GatewayRunner into
gateway/authz_mixin.py:GatewayAuthorizationMixin. Behavior-neutral; gateway/run.py
16200 -> 15812 LOC.
Methods moved (~389 LOC): _is_user_authorized, _get_unauthorized_dm_behavior,
_adapter_dm_policy, _adapter_enforces_own_access_policy. The two adapter-policy
helpers are private to _is_user_authorized, so the cluster is fully self-contained
(zero outside-cluster self.method calls after the lift). All self.* calls resolve
unchanged via the MRO (GatewayRunner(GatewayAuthorizationMixin, ...)).
Import split: 6 neutral deps (os, Optional, Platform, SessionSource, the two
whatsapp_identity helpers) at the mixin module top; the module-level logger is
imported lazily inside _is_user_authorized (from gateway.run import logger) so
the mixin never imports gateway.run at module scope -> no cycle. The lazy import
preserves the exact logger name (gateway.run) so log records are unchanged.
a706a349b5