The adapter fix decoded `'["-100","-200"]'` before comma-splitting, but
the runner's central gate in gateway/authz_mixin.py::_coerce_allow_set
reads the same YAML-bridged env chain (TELEGRAM_GROUP_ALLOWED_CHATS,
TELEGRAM_ALLOWED_USERS via _auth_env) and still produced
{'["1"', '"2"]'}, so a group message admitted by the adapter could
still be rejected upstream.
Move the decoder to gateway/platforms/_shared.py, which both the adapter
and authz_mixin already import from (no plugin -> gateway cycle), and
route _coerce_allow_set through it. One invariant test on the runner
side, red before this change.
122ad719b5