docs: Enable Ruff formatting of Python code blocks in Markdown

[vdusek]

What & why

Ruff 0.16 formats Python code blocks embedded in Markdown files, and does so by default. In this repo the feature was inert because [tool.ruff] include was an allowlist of *.py globs, so no Markdown file ever reached the formatter. Adding **/*.md and **/*.mdx turns it on, which means docs snippets are now held to the same formatting standard as the rest of the codebase instead of drifting by hand.

MDX needs the explicit extension = { mdx = "markdown" } mapping - without it Ruff would try to parse .mdx as Python and fail. Most of the Python snippets in docs/ live in .mdx, so the mapping is what makes this useful here.

No changes to the lint / format Poe tasks are needed. lint already runs ruff format --check and format already runs ruff format, so both pick Markdown up automatically. Note that ruff check (lint rules) does not support Markdown yet, so only formatting is enforced there.

Notes

• Code blocks under docs/ are formatted to 90 columns, not 120, because of the existing docs/pyproject.toml override that keeps doc snippets free of a horizontal scrollbar. That is why one line in the upgrading guide got wrapped.
• The second commit collapses the include allowlist to a single **/*.py glob. This is a pure simplification: no tracked .py file lives outside the previously listed directories, and Ruff processes exactly the same 189 files before and after.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.62%. Comparing base (b49fb35) to head (ee18c40).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #982   +/-   ##
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  Coverage   94.62%   94.62%           
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  Files          58       58           
  Lines        5248     5248           
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  Hits         4966     4966           
  Misses        282      282           

Flag	Coverage Δ
integration	92.28% <ø> (+0.01%)	⬆️
unit	83.89% <ø> (ø)

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