Introduce a reusable helper that produces upload-ready blobs for
adapters that share the same image-asset semantics (dongchedi,
sohuhao, wangyihao, weixin-gzh, zol). Centralizes URL candidate
expansion and passthrough mime detection so each adapter focuses on
its own publish protocol.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
