Improve parametrize error for invalid tuple-style values

[itscloud0]

Closes #14619.

Summary

This improves the collection error raised when tuple-style parametrize argument names receive an invalid scalar parameter set.

Previously, @pytest.mark.parametrize("x,", [None, "foo", "bar"]) crashed while formatting the validation error because pytest called len() on None. The updated validation reports the bad parameter set index directly instead.

It also makes the existing string mismatch case clearer by displaying the string as the sequence pytest counted, e.g. ['f', 'o', 'o'], instead of foo.

Test plan

• uv run --python /opt/homebrew/bin/python3.12 --extra dev --with-editable . python -m pytest testing/python/metafunc.py::TestMetafunc::test_parametrize_trailing_comma_scalar_parameter_set testing/python/metafunc.py::TestMetafunc::test_parametrize_string_parameter_set_error_shows_sequence -q
• uv run --python /opt/homebrew/bin/python3.12 --extra dev --with-editable . python -m pytest testing/python/metafunc.py -q
• uvx pre-commit run --files src/_pytest/mark/structures.py testing/python/metafunc.py AUTHORS changelog/14619.bugfix.rst

Notes

Full test suite was not run locally; this change is covered by the focused parametrization tests and the full testing/python/metafunc.py module.

AI tooling was used while preparing this patch, and the commit includes a Co-authored-by trailer for attribution.