Fix module fixtures with --doctest-modules

[minbang930]

Summary

• Fix fixture registration when --doctest-modules collects a Python module before the normal module collector.
• Key parsed fixture holders by their visibility anchor, so normal tests get module-scoped FixtureDefs without relaxing node-based scoping.
• Add a regression test, changelog entry, and AUTHORS entry.

Closes #14533.

Test plan

• PYTHONPATH=src python /tmp/pytest-14533-repro/run_checkout_pytest.py -q testing/test_doctest.py::TestDoctests::test_module_fixture_available_to_normal_test_with_doctestmodules testing/test_doctest.py::TestDoctests::test_doctestmodule_with_fixtures testing/test_doctest.py::TestDoctests::test_doctestmodule_three_tests testing/test_doctest.py::TestDoctestAutoUseFixtures — 37 passed in 0.75s
• PYTHONPATH=src python /tmp/pytest-14533-repro/run_checkout_pytest.py -q testing/test_conftest.py::test_conftest_fixture_scoping_with_testpaths_outside_rootdir testing/test_conftest.py::test_conftest_fixture_from_ancestor_above_rootdir testing/deprecated_test.py::TestFixtureNodeidDeprecations — 6 passed in 0.14s
• PYTHONPATH=src python /tmp/pytest-14533-repro/run_checkout_pytest.py -q testing/python/fixtures.py::TestFixtureManagerParseFactories::test_parsefactories_conftest testing/python/fixtures.py::TestFixtureManagerParseFactories::test_parsefactories_conftest_and_module_and_class testing/python/fixtures.py::TestFixtureManagerParseFactories::test_parsefactories_relative_node_ids — 3 passed in 0.10s
• git diff --check
• python -m compileall -q src/_pytest/fixtures.py testing/test_doctest.py
• python -m ruff check src/_pytest/fixtures.py testing/test_doctest.py
• python -m ruff format --check src/_pytest/fixtures.py testing/test_doctest.py

AI assistance

OpenAI Codex helped reproduce the bug, draft the regression test, and prepare the initial patch. I reviewed the diff, checked the fixture scoping risk, added the changelog and AUTHORS entries, reran the targeted tests, and take responsibility for the change.

• Include new tests or update existing tests when applicable.
• Allow maintainers to push and squash when merging my commits.
• Add closes #14533 to the PR description and commit message.
• Credit the AI agent in a Co-authored-by commit trailer.
• Create a changelog file in changelog/.
• Add myself to AUTHORS in alphabetical order.