Fix phantom nodeid for tests collected outside rootpath (#13254)

[mnajaf]

Fixes #13254.

Problem

When pytest collects tests from a path that lives outside rootpath
(for example a pytest.ini inside tests/a/aa/ while an additional
test file sits at tests/b/bb/test_b.py), the displayed node ID for
the out-of-rootpath test is a synthesized path that does not exist on
disk.

Minimal reproducer:

tests/
├── a/aa/pytest.ini
├── a/aa/test_a.py
└── b/bb/test_b.py

Run from tests/a/:

pytest ./aa ../b/bb -vv

Before this change:

aa\test_a.py::test_a PASSED
aa\test_b.py::test_b <- ..\..\b\bb\test_b.py PASSED

aa\test_b.py does not exist; the file is actually at ../../b/bb/test_b.py.

Cause

Node.__init__ in src/_pytest/nodes.py falls back to
_check_initialpaths_for_relpath when the collected path cannot be
expressed relative to rootpath. That yields a bare node ID like
test_b.py::test_b. Config.cwd_relative_nodeid then assumed every
node ID was rootpath-relative and rebuilt the display string as
rootpath / base_path_part, which in this case points nowhere.

Fix

Check whether the synthesized fullpath actually exists on disk. If
it does not, the node ID was resolved via the initial paths (not
rootpath), so return it unchanged. The <- ... location hint rendered
by the terminal reporter continues to show the real file location, so
the correct path information is still presented without the misleading
prefix.

After this change the same invocation produces:

aa\test_a.py::test_a PASSED
test_b.py::test_b <- ..\..\b\bb\test_b.py PASSED

Tests

Added TestNodeIDHandling::test_nodeid_outside_rootpath_not_synthesized
in testing/test_terminal.py. The test reproduces the issue layout
and asserts that the displayed node ID for the out-of-rootpath test
does not gain a phantom aa/ prefix. The full testing/test_config.py
and testing/test_terminal.py suites pass locally on Windows
(Python 3.13) alongside testing/test_collection.py,
testing/test_session.py, testing/test_nodes.py,
testing/test_reports.py, and testing/test_skipping.py
(841 passed total).

Notes

This is my first contribution to pytest; happy to adjust the comment
wording, changelog phrasing, or test placement if preferred.