feat(gateway): add WeCom (Enterprise WeChat) platform support (#3847)

Adds WeCom as a gateway platform adapter using the AI Bot WebSocket
gateway for real-time bidirectional communication. No public endpoint
or new pip dependencies needed (uses existing aiohttp + httpx).

Features:
- WebSocket persistent connection with auto-reconnect (exponential backoff)
- DM and group messaging with configurable access policies
- Media upload/download with AES decryption for encrypted attachments
- Markdown rendering, quote context preservation
- Proactive + passive reply message modes
- Chunked media upload pipeline (512KB chunks)

Cherry-picked from PR #1898 by EvilRan with:
- Moved to current main (PR was 300 commits behind)
- Skipped base.py regressions (reply_to additions are good but belong
  in a separate PR since they affect all platforms)
- Fixed test assertions to match current base class send() signature
  (reply_to=None kwarg now explicit)
- All 16 integration points added surgically to current main
- No new pip dependencies (aiohttp + httpx already installed)

Fixes #1898

Co-authored-by: EvilRan <EvilRan@users.noreply.github.com>

tests/gateway/test_unauthorized_dm_behavior.py

@@ -20,7 +20,7 @@ def _clear_auth_env(monkeypatch) -> None:
         "SMS_ALLOWED_USERS",
         "MATTERMOST_ALLOWED_USERS",
         "MATRIX_ALLOWED_USERS",
-        "DINGTALK_ALLOWED_USERS", "FEISHU_ALLOWED_USERS",
+        "DINGTALK_ALLOWED_USERS", "FEISHU_ALLOWED_USERS", "WECOM_ALLOWED_USERS",
         "GATEWAY_ALLOWED_USERS",
         "TELEGRAM_ALLOW_ALL_USERS",
         "DISCORD_ALLOW_ALL_USERS",
@@ -31,7 +31,7 @@ def _clear_auth_env(monkeypatch) -> None:
         "SMS_ALLOW_ALL_USERS",
         "MATTERMOST_ALLOW_ALL_USERS",
         "MATRIX_ALLOW_ALL_USERS",
-        "DINGTALK_ALLOW_ALL_USERS", "FEISHU_ALLOW_ALL_USERS",
+        "DINGTALK_ALLOW_ALL_USERS", "FEISHU_ALLOW_ALL_USERS", "WECOM_ALLOW_ALL_USERS",
         "GATEWAY_ALLOW_ALL_USERS",
     ):
         monkeypatch.delenv(key, raising=False)