feat(channel): render post attachment zone in normalize

[yjhcjykwbk-jlsec]

Summary

The rich-text attachment zone (top-level files array on a post message) was ignored by the post converter: only the locale document (title/content/content_v2) was flattened and scanned for resources, so attachments attached to a rich-text message were invisible to channel consumers.

This PR makes the attachment zone visible and downloadable (node-aligned with larksuite/channel-sdk-node):

• lark_channel/channel/normalize/converters/post.py
  • _attachment_files(): extract the top-level files: [{file_key, file_name, is_folder}] array
  • render attachment-zone entries as <file key="..." name="..."/> / <folder key="..." name="..."/> lines after the body, matching the standalone file/folder converters
  • surface attachment-zone files as ResourceDescriptor(type="file") so they are downloadable; folders stay tag-only (mirrors the standalone folder converter's resources=[])
  • the attachment zone is also included in convert_body (i.e. InboundMessage.body_text)
• lark_channel/channel/tests/test_flatten.py: three new tests — rendering + resource extraction, empty files array tolerance, and body_text propagation

Wire shape

{
  "zh_cn": { "title": "报告", "content": [[{"tag": "text", "text": "正文"}]] },
  "files": [
    { "file_key": "file_a", "file_name": "report.pdf" },
    { "file_key": "dir_1", "file_name": "assets", "is_folder": true }
  ]
}

renders as:

# 报告
正文
<file key="file_a" name="report.pdf"/>
<folder key="dir_1" name="assets"/>

with resources: [ResourceDescriptor(type="file", file_key="file_a", file_name="report.pdf")].

Test

python3 -m pytest lark_channel/channel/tests/ -q
# 893 passed (incl. 3 new)