[codex] Add WeChat public account article converter

[Lego1997]

Summary

• Add a dedicated WeChat public account article converter for mp.weixin.qq.com/s/... pages.
• Use browser-like request headers for WeChat article URLs so the CLI can fetch article HTML instead of the platform verification page.
• Preserve article title, account metadata, cover image, body content, and in-article images while omitting common WeChat platform chrome.
• Document direct web page conversion and WeChat article behavior in the README.

Validation

• uv run --with pytest pytest -q 'tests/test_module_vectors.py::test_guess_stream_info[test_vector8]' 'tests/test_module_vectors.py::test_convert_local[test_vector8]' 'tests/test_module_vectors.py::test_convert_stream_with_hints[test_vector8]' 'tests/test_module_vectors.py::test_convert_stream_without_hints[test_vector8]' 'tests/test_module_vectors.py::test_convert_file_uri[test_vector8]' 'tests/test_module_vectors.py::test_convert_data_uri[test_vector8]' tests/test_module_misc.py::test_wechat_uri_uses_browser_like_headers
• uv run markitdown 'https://mp.weixin.qq.com/s/dQPd5jgGobw-Q4LxLq3wHA' -o /tmp/upstream-wechat-live.md
• uvx black==23.7.0 --check packages/markitdown/src/markitdown/converters/_wechat_converter.py packages/markitdown/src/markitdown/converters/__init__.py packages/markitdown/src/markitdown/_markitdown.py packages/markitdown/tests/_test_vectors.py packages/markitdown/tests/test_module_misc.py
• uvx pre-commit run --all-files

[Lego1997]

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