Accept MarkDecorator as get_closest_marker default

[ace2016]

closes #13354

Problem

Node.get_closest_marker(name, default) documents and type-hints its default as a Mark:

@overload
def get_closest_marker(self, name: str, default: Mark) -> Mark: ...

But Mark.__init__ is private-gated — it ends in check_ispytest(_ispytest), which raises a PytestDeprecationWarning ("A private pytest class or function was used.") when a user constructs a Mark directly. That leaves the public parameter with no supported way to produce a value: the API asks for a Mark, and the obvious way to build one warns.

Approach

So get_closest_marker now also accepts a MarkDecorator and unwraps it:

if isinstance(default, MarkDecorator):
    default = default.mark

# now works, no warning
item.get_closest_marker("foo", pytest.mark.foo(1))

pytest.mark.foo(1) builds its Mark internally with _ispytest=True, so nothing warns.

Changes

File	Change
src/_pytest/nodes.py	Widen default to Mark | MarkDecorator, unwrap to .mark, document it
testing/test_mark.py	Regression test alongside the existing test_mark_closest
changelog/13354.improvement.rst	Changelog entry
AUTHORS	Added myself

doc/en/reference/reference.rst needed no edit — it autodocs the method, so the updated docstring flows through.

Testing

• testing/test_mark.py and testing/test_nodes.py pass (124 passed, 1 xfailed).
• The new test is a genuine regression test: reverting only the nodes.py change makes it fail, restoring it makes it pass.
• Full pre-commit on all changed files passes, including mypy, ruff, ruff format, and codespell.

The new test uses the registered dummy marker foo from pyproject.toml rather than an ad-hoc name, because the decorator is constructed in the outer test process where pytest's own suite runs with --strict-markers.

##Checklist:

• Include documentation when adding new features.
  -> The method docstring is updated; reference docs autodoc it.
• Include new tests or update existing tests when applicable.
• Allow maintainers to push and squash when merging my commits.
• Add text like closes #XYZW to the PR description and/or commits.
• If AI agents were used, they are credited in Co-authored-by commit trailers.
• Create a new changelog file in the changelog directory.
• Add yourself to AUTHORS in alphabetical order.

[ace2016]

@RonnyPfannschmidt could you please take a look when you have the time? thank you