fix(gateway): add trust_env=True to aiohttp sessions in SMS, Slack, Teams, Google Chat adapters

aiohttp.ClientSession defaults to trust_env=False, which silently ignores
HTTP_PROXY, HTTPS_PROXY, and ALL_PROXY environment variables. Users behind
a corporate or network proxy cannot reach external APIs on any of these
platforms — all outbound requests fail with connection errors.

Symmetric with wecom.py (line 276), weixin.py (lines 1055/1268/1274), and
matrix.py (no-proxy path) which already set this flag. Complements the
open LINE fix (#26635) with the remaining gateway and plugin adapters.

Changed:
- gateway/platforms/sms.py: persistent Twilio session (connect) + fallback
  session (send) — both hit https://api.twilio.com
- gateway/platforms/slack.py: ephemeral response_url POST session —
  hits https://hooks.slack.com/... callback URLs
- plugins/platforms/teams/adapter.py: standalone send session —
  hits login.microsoftonline.com (token) + Bot Framework service URL
- plugins/platforms/google_chat/adapter.py: standalone send session —
  hits https://chat.googleapis.com/v1/...

WhatsApp sessions are excluded: they connect to http://127.0.0.1:{port}
(local bridge) and must not be routed through a system proxy.

plugins/platforms/google_chat/adapter.py

@@ -3246,7 +3246,7 @@ async def _standalone_send(
         return {"error": "Google Chat standalone send: aiohttp not installed"}
 
     try:
-        async with _aiohttp.ClientSession(timeout=_aiohttp.ClientTimeout(total=30.0)) as session:
+        async with _aiohttp.ClientSession(timeout=_aiohttp.ClientTimeout(total=30.0), trust_env=True) as session:
             async with session.post(
                 url,
                 json=body,