fix: add to_plain_md by datajson.py & unit

[renpengli01]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##              dev     #550      +/-   ##
==========================================
+ Coverage   90.97%   91.02%   +0.04%     
==========================================
  Files         102      106       +4     
  Lines        8890     9173     +283     
==========================================
+ Hits         8088     8350     +262     
- Misses        802      823      +21     

Files with missing lines	Coverage Δ
llm_web_kit/input/datajson.py	85.78% <100.00%> (+1.09%)	⬆️
llm_web_kit/libs/doc_element_type.py	100.00% <100.00%> (ø)
llm_web_kit/simple.py	89.15% <100.00%> (-7.67%)	⬇️

... and 4 files with indirect coverage changes

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[drunkpig]

exclude_inline_types应该包含 行内公式和行内代码

[drunkpig]

VIDEO， AUDIO也应该在内

[drunkpig]

这个测试用例应当包含我说的那2个case.
从原始的html来时输出。html放到文件里，容易维护。这个html可以手工构造的。包含以上各种元素。

[drunkpig]

这种写法不易维护，字符串坨在一起