Fix node IDs for files outside rootdir

[marko1olo]

Fixes #9703.

When -c config/pytest.ini makes config/ the rootdir, explicitly requested test files outside that rootdir currently receive empty filesystem collector node IDs. That can collapse different files with the same test name into the same cache key, for example ::test_same, and makes file-scoped fixture visibility depend on a broken parent node ID.

This changes outside-root filesystem collectors to use a stable path relative to rootdir, such as ../test/test_file1.py, instead of an empty node ID. Terminal display still renders paths relative to the invocation directory, so users see test/test_file1.py::... in output.

Checks run locally:

• python -m pytest testing/test_collection.py::TestCollectFS::test_config_outside_test_paths_keeps_unique_lastfailed_nodeids testing/test_collection.py::TestCollectFS::test_config_outside_test_paths_keeps_file_scoped_autouse -q
• python -m pytest testing/test_collection.py::TestCollectFS -q
• python -m pytest testing/test_conftest.py::test_conftest_fixture_scoping_with_testpaths_outside_rootdir testing/test_conftest.py::test_conftest_fixture_from_ancestor_above_rootdir -q
• python -m pytest testing/test_cacheprovider.py::TestLastFailed::test_lastfailed_usecase testing/test_cacheprovider.py::TestLastFailed::test_lastfailed_difference_invocations testing/test_cacheprovider.py::TestLastFailed::test_lastfailed_args_with_deselected testing/test_cacheprovider.py::TestLastFailed::test_lastfailed_collectfailure testing/test_cacheprovider.py::TestLastFailed::test_lastfailed_with_known_failures_not_being_selected -q
• python -m ruff check src/_pytest/nodes.py src/_pytest/config/__init__.py testing/test_collection.py
• python -m ruff format --check src/_pytest/nodes.py src/_pytest/config/__init__.py testing/test_collection.py
• git diff --check