Send agent-generated files as native WeChat attachments via the iLink upload flow, and fix the wire protocol for file uploads: dedicated wire ObjectMapper (bypasses the global Long-to-String serializer), md5/len fields and encrypt_type on media items, channel_version 1.0.2, and explicit business-error handling on ret != 0. The weixin adapter now routes generated-file URLs through GeneratedFileScrubber, matching WeCom/Feishu behavior.
Fixes#307