WechatChannel made synchronous filesystem calls (mkdir, write_text,
read_bytes, Path.replace, unlink) directly inside async entry points:
_poll_loop, _bind_via_qrcode, _ensure_authenticated, _extract_image_file,
_extract_file_item, start, _send_image_attachment, _send_file_attachment.
Under slow disks, large files, or concurrent load these blocked the
asyncio event loop and stalled the channel worker.
Construction was also blocking: __init__ called _load_state() (os.stat +
read_text) synchronously, and ChannelService._start_channel() instantiates
the channel directly on the async path, so constructing WechatChannel in an
async context raised BlockingError. Persisted state (auth token + cursor)
is now loaded in start() via asyncio.to_thread, leaving __init__ IO-free.
Offload each call to a thread via asyncio.to_thread, matching the existing
pattern in channels/manager.py and dingtalk.py. The sync helpers
(_save_state, _save_auth_state, _load_auth_state, _stage_downloaded_file)
keep their signatures; only the async call sites wrap them.
Adds tests/blocking_io/test_wechat_channel_state.py as a regression anchor
covering the IO-free constructor (the production _start_channel path), the
staging write path, and the auth-state read path. Detected by
`make detect-blocking-io`.
