Avoid traceback formatting for xfail(run=False)

[cjc0013]

Refs #9436.

What changed

• Tag the internal xfail(run=False) exception raised before a test body is executed.
• Use that tag during report creation to keep longrepr as the [NOTRUN] reason string instead of formatting a traceback.
• Leave normal failures, imperative pytest.xfail(), and other xfail paths using the existing report formatting path.
• Add a regression test that makes traceback formatting fail if it is called for xfail(run=False).

Validation

python -m pytest testing/test_skipping.py -k "xfail_not_run" -q
# 3 passed, 90 deselected

python -m pytest testing/test_skipping.py -q
# 93 passed

python -m pytest testing/test_reports.py testing/test_junitxml.py -q
# 158 passed, 2 skipped

python -m ruff format --check src/_pytest/reports.py src/_pytest/skipping.py testing/test_skipping.py
python -m ruff check src/_pytest/reports.py src/_pytest/skipping.py testing/test_skipping.py
# passed

[RonnyPfannschmidt]

Closing as ai slop without human oversight