fix(gateway/weixin): split poll/send sessions, reuse live adapter for cron & send_message

- gateway/platforms/weixin.py:
  - Split aiohttp.ClientSession into _poll_session and _send_session
  - Add _LIVE_ADAPTERS registry so send_weixin_direct() reuses the connected gateway adapter instead of creating a competing session
  - Fixes silent message loss when gateway is running (iLink token contention)

- cron/scheduler.py:
  - Support comma-separated deliver values (e.g. 'feishu,weixin') for multi-target delivery
  - Delay pconfig/enabled check until standalone fallback so live adapters work even when platform is not in gateway config

- tools/send_message_tool.py:
  - Synthesize PlatformConfig from WEIXIN_* env vars when gateway config lacks a weixin entry
  - Fall back to WEIXIN_HOME_CHANNEL env var for home channel resolution

- tests/gateway/test_weixin.py:
  - Update mocks to include _send_session

tests/gateway/test_weixin.py

@@ -311,6 +311,7 @@ class TestWeixinChunkDelivery:
     def _connected_adapter(self) -> WeixinAdapter:
         adapter = _make_adapter()
         adapter._session = object()
+        adapter._send_session = adapter._session
         adapter._token = "test-token"
         adapter._base_url = "https://weixin.example.com"
         adapter._token_store.get = lambda account_id, chat_id: "ctx-token"
@@ -420,6 +421,7 @@ class TestWeixinBlankMessagePrevention:
     def test_send_empty_content_does_not_call_send_message(self, send_message_mock):
         adapter = _make_adapter()
         adapter._session = object()
+        adapter._send_session = adapter._session
         adapter._token = "test-token"
         adapter._base_url = "https://weixin.example.com"
         adapter._token_store.get = lambda account_id, chat_id: "ctx-token"