feat(gateway): one gateway.trust_env key controls aiohttp proxy-env honoring at every adapter site (#48820 bug 3)

Every gateway/plugin platform adapter hard-coded aiohttp.ClientSession(trust_env=True)
(~20 sites), so a gateway launched by a Windows Scheduled Task that inherits a stale
HTTP_PROXY (Clash/V2Ray on 127.0.0.1:7890) looped on 'Cannot connect to host' with no
way to opt out short of NO_PROXY hacks per vendor host.

- gateway/platforms/base.py: gateway_trust_env() reads gateway.trust_env (default true);
  resolve_proxy_url() skips generic HTTP(S)_PROXY/ALL_PROXY + macOS system-proxy
  auto-detect when false (explicit per-platform vars still win).
- All aiohttp ClientSession sites in weixin, qqbot, matrix, line, wecom, slack, sms,
  teams, google_chat now pass trust_env=gateway_trust_env(); mattermost + homeassistant
  bare sessions gain the same kwarg (intent of #70119 / #56229).
- DEFAULT_CONFIG + cli-config.yaml.example + messaging docs.
- tests/gateway/test_gateway_trust_env.py: config flip + no-bare-literal sweep.

Reported-by: @ranlingfeng (#48820), @frontnopipe-cloud (#76309)
Co-authored-by: rcarrata <rcarratalasanchez@gmail.com>
Co-authored-by: Backroads4Me <TEDLANHAM@GMAIL.COM>

plugins/platforms/teams/adapter.py

@@ -103,6 +103,7 @@ TextBlock = None  # type: ignore[assignment,misc]
 from gateway.config import Platform, PlatformConfig
 from gateway.platforms.helpers import MessageDeduplicator
 from gateway.platforms.base import (
+    gateway_trust_env,
     BasePlatformAdapter,
     MessageEvent,
     MessageType,
@@ -641,7 +642,7 @@ async def _standalone_send(
         # Per-request timeouts so a slow STS endpoint cannot starve the
         # subsequent activity POST of its budget.
         per_request_timeout = _aiohttp.ClientTimeout(total=15.0)
-        async with _aiohttp.ClientSession(trust_env=True) as session:
+        async with _aiohttp.ClientSession(trust_env=gateway_trust_env()) as session:
             async with session.post(
                 token_url,
                 data={