fix(telegram): runner-side allowlist gate decodes JSON list strings too

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.

plugins/platforms/telegram/adapter.py

@@ -18,7 +18,11 @@ from hermes_cli import setup_platforms
 logger = logging.getLogger(__name__)
 
 from agent.deadline import run_bounded_async
-from gateway.platforms._shared import get_scoped_secret as _get_scoped_secret, platform_gate_env as _scoped_gate_env
+from gateway.platforms._shared import (
+    decode_json_list_literal as _decode_json_list_literal,
+    get_scoped_secret as _get_scoped_secret,
+    platform_gate_env as _scoped_gate_env,
+)
 
 
 def _redact_telegram_error_text(error: object) -> str:
@@ -33,23 +37,6 @@ def _redact_telegram_error_text(error: object) -> str:
         return "<telegram error redacted>"
 
 
-def _decode_json_list_literal(raw):
-    """Decode a JSON-encoded allowlist written by ``hermes config set``.
-
-    String-typed defaults keep list literals verbatim on write (``allowed_chats`` is
-    declared as ``""``), so the config can hold ``'["-100","-200"]'`` as a string.
-    Malformed JSON passes through unchanged and keeps the legacy comma-split path.
-    """
-    if isinstance(raw, str) and raw.lstrip()[:1] == "[":
-        try:
-            loaded = json.loads(raw)
-        except ValueError:
-            return raw
-        if isinstance(loaded, list):
-            return loaded
-    return raw
-
-
 def _consume_abandoned_task(task: asyncio.Task) -> None:
     """Observe a detached task's terminal exception to avoid noisy loop logs."""
     try: