feat(gateway): add WeCom callback-mode adapter for self-built apps

Add a second WeCom integration mode for regular enterprise self-built
applications.  Unlike the existing bot/websocket adapter (wecom.py),
this handles WeCom's standard callback flow: WeCom POSTs encrypted XML
to an HTTP endpoint, the adapter decrypts, queues for the agent, and
immediately acknowledges.  The agent's reply is delivered proactively
via the message/send API.

Key design choice: always acknowledge immediately and use proactive
send — agent sessions take 3-30 minutes, so the 5-second inline reply
window is never useful.  The original PR's Future/pending-reply
machinery was removed in favour of this simpler architecture.

Features:
- AES-CBC encrypt/decrypt (BizMsgCrypt-compatible)
- Multi-app routing scoped by corp_id:user_id
- Legacy bare user_id fallback for backward compat
- Access-token management with auto-refresh
- WECOM_CALLBACK_* env var overrides
- Port-in-use pre-check before binding
- Health endpoint at /health

Salvaged from PR #7774 by @chqchshj.  Simplified by removing the
inline reply Future system and fixing: secrets.choice for nonce
generation, immediate plain-text acknowledgment (not encrypted XML
containing 'success'), and initial token refresh error handling.

gateway/platforms/webhook.py

@@ -201,6 +201,7 @@ class WebhookAdapter(BasePlatformAdapter):
             "dingtalk",
             "feishu",
             "wecom",
+            "wecom_callback",
             "weixin",
             "bluebubbles",
         ):