build(test): declare the markdown test dependency

[JessYanCoding]

Summary

• declare markdown (Python-Markdown) in the test optional-dependency group

tests/unittest/test_render_html_table.py:1 does import markdown, but the package is not
declared anywhere in pyproject.toml — the only occurrence of the string "markdown" in that file
is inside [project].keywords. A pytest collection error is fatal to the whole run, so on a clean
.[test] environment the entire suite is skipped:

$ python -m pytest tests/unittest --no-cov
collected 2889 items / 1 error
E   ModuleNotFoundError: No module named 'markdown'
!!!!!!!! Interrupted: 1 error during collection !!!!!!!!
$ echo $?
2

This is not visible in CI because .github/workflows/cli.yml runs coverage run, whose
command_line targets a single file rather than the suite. docs/requirements.txt carries
markdown-gfm-admonition for the mkdocs build, but that file is not part of the test environment.

Validation

• uv pip install --dry-run -e ".[test]" on this branch resolves markdown==3.10.3; on
  next @ 04390cd it resolves nothing matching markdown
• with the dependency present, python -m pytest tests/unittest --no-cov collects and executes
  2889 tests instead of exiting 2 at collection (remaining failures are pre-existing on next and
  unrelated to this change)
• no source or test files touched, so there is nothing to regress

🤖 Generated with Claude Code

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[JessYanCoding]

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