fix: resolve doctest locations for fixture definitions

[vetrovk]

Fixes #14700.

Summary

Starting with pytest 8.4, @pytest.fixture returns a FixtureFunctionDefinition. On Python 3.12, doctest line discovery could return None for that wrapper. A skipped fixture doctest then caused an internal error during report creation.

This reuses the existing wrapper-unwrapping behavior before doctest line lookup. Report generation and assertions in reports.py remain unchanged.

Validation

• tox -e py312 -- testing/test_doctest.py
• tox -e py312 -- testing/test_doctest.py::TestDoctests::test_fixture_doctest_skip_has_line_number
• pre-commit run --files src/_pytest/doctest.py testing/test_doctest.py changelog/14700.bugfix.rst
• git diff --check
• Manual fixture-doctest reproducer and the original jaraco.test scenario on Python 3.12

Checklist

• Include documentation when adding new features.
• Include new tests or update existing tests when applicable.
• Allow maintainers to push and squash when merging my commits.
• Add text like Fixes #14700 to the PR description and/or commits.
• AI agents were used and are credited in the commit's Co-authored-by trailer.
• Create a new changelog file in the changelog directory.