fix: salvage follow-ups for Weixin adapter (#6747)

- Remove sys.path.insert hack (leftover from standalone dev)
- Add token lock (acquire_scoped_lock/release_scoped_lock) in
  connect()/disconnect() to prevent duplicate pollers across profiles
- Fix get_connected_platforms: WEIXIN check must precede generic
  token/api_key check (requires both token AND account_id)
- Add WEIXIN_HOME_CHANNEL_NAME to _EXTRA_ENV_KEYS
- Add gateway setup wizard with QR login flow
- Add platform status check for partially configured state
- Add weixin.md docs page with full adapter documentation
- Update environment-variables.md reference with all 11 env vars
- Update sidebars.ts to include weixin docs page
- Wire all gateway integration points onto current main

Salvaged from PR #6747 by Zihan Huang.

website/sidebars.ts

@@ -108,6 +108,7 @@ const sidebars: SidebarsConfig = {
         'user-guide/messaging/dingtalk',
         'user-guide/messaging/feishu',
         'user-guide/messaging/wecom',
+        'user-guide/messaging/weixin',
         'user-guide/messaging/bluebubbles',
         'user-guide/messaging/open-webui',
         'user-guide/messaging/webhooks',