feat(im/convert): support content_v2 blocks in post message conversion

[liujiashu-shiro]

Summary

This change adds support for converting content_v2 blocks in post messages and changes the default Markdown conversion mode from structured to native, preserving Feishu/Lark client-native Markdown rendering.

Changes

• Prefer content_v2 when flattening PostContent, with fallback to legacy content
• Post-process tag: md text to resolve mentions, extract Markdown image resources, and preserve paired code-fence contents
• Change the default MarkdownConverter.tag_md_mode and markdown_to_post_ast mode to native
• Update sender, Markdown conversion, media caption, and native mode tests for the new default behavior
• Add regression coverage for content_v2, mentions, image resources, and code-fence handling

Test Plan

• Unit tests pass
• Manually verify the lark xxx command works locally
• Verify post messages with content_v2 Markdown, mentions, and images convert correctly

Related Issues

N/A