Add WeChat ClawBot connector

[jasmine889966]

Summary

Adds a two-way WeChat ClawBot connector on Tencent's iLink API (ilinkai.weixin.qq.com), so DMs on your phone can reach a local OpenWorker session.

• QR login in the Connectors UI (POST /v1/connectors/weixin/qrcode + status poll); server turns the scan payload into a PNG data URL
• Inbound long-poll via getupdates; outbound through send_message with a per-chat context_token
• The account that confirms the QR is pre-added to the allow-list (same idea as Slack's installer)
• Enabling WeChat / Telegram / Slack on a session claims it as the DM route when none is set yet

v1: DMs only (no groups). Outbound needs a prior inbound message (WeChat's ~24h window).

Test plan

• Unit + integration: pytest tests/test_weixin_flow.py tests/test_weixin_ilink.py tests/test_connectors.py tests/test_connectors_allowlist.py tests/test_connector_registry.py tests/test_connections.py tests/test_gateway_inbox_reply.py tests/test_message_source.py — 93 passed
• npx tsc --noEmit in surfaces/gui
• QR login REST → PNG data URL → confirmed saves profile + allow-list (test_qr_login_rest_connects_and_preallows_scanner; live /v1/connectors/weixin/qrcode returns a valid PNG)
• Enable WeChat on a session claims DM route and inbound DM is delivered (test_enable_weixin_claims_dm_route_and_delivers)
• send_message to weixin:… uses stored context_token (test_send_message_weixin_uses_context_token)
• Disconnect clears context tokens (test_disconnect_clears_context_tokens)