Skip Sphinx _build/ output during doctest collection

[tony]

Summary

• Fix: pytest_doctest_docutils no longer collects Sphinx _build/ output. Built docs mirror their sources — including relative {include} directives that don't resolve from the build tree — so once docs/_build/ existed, collecting those copies aborted the entire test session with a docutils SystemMessage.
• Root cause: pytest's default norecursedirs excludes build but not _build (the conventional Sphinx output dir). The plugin's *.md/*.rst collection globs then matched the build artifacts.
• Approach: add a pytest_ignore_collect hook that skips any path with a _build component — stopping both directory recursion and file collection, so every consumer is fixed with no per-project config.

Behavior

Before — with docs/_build/ present, collection aborts:

ERROR docs/_build/history.md - docutils.utils.SystemMessage: (SEVERE/4)
Directive "include": file not found: '.../docs/_build/../CHANGES'
!!! Interrupted: 1 error during collection !!!

After — _build/ is ignored and collection proceeds normally.

Design decisions

• Fix in the plugin, not per-project config: _build/ could be excluded per-repo via norecursedirs or collect_ignore_glob, but this plugin is shared across the git-pull projects and the failure is universal. Handling it once in pytest_ignore_collect fixes every consumer with no config churn — and mirrors how pytest already excludes build by default.
• Match any _build path component: covers both docs/_build/history.md and the nested docs/_build/html/history.md layout, and skips the directory itself (halting recursion) as well as individual files.

Test plan

• test_ignore_build_artifacts[build-root] — a docs/_build/history.md with an unresolvable include is skipped while the genuine source doctest still collects
• test_ignore_build_artifacts[build-html-subdir] — same guarantee for the nested docs/_build/html/ layout
• Full suite passes with docs/_build/ populated on disk (the reported failure condition)
• uv run ruff check . and uv run ruff format .
• uv run mypy src tests
• uv run py.test
• just build-docs

Verification

Reproduce the original failure condition and confirm it now passes:

$ just build-docs && uv run py.test

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.59%. Comparing base (bfd6dd1) to head (27d9736).

Files with missing lines	Patch %	Lines
src/pytest_doctest_docutils.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
+ Coverage   73.08%   73.59%   +0.51%     
==========================================
  Files          14       14              
  Lines         836      856      +20     
==========================================
+ Hits          611      630      +19     
- Misses        225      226       +1     

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