-k expression ignores directory paths

• a detailed description of the bug or problem you are having

The -k expression does not work with directory paths. For example, pytest -k '/common' does not collect any tests from the /common folder.

• output of pip list from the virtual environment you are using

Package    Version
---------- -------
attrs      25.4.0
iniconfig  2.3.0
packaging  26.0
pip        25.3
pluggy     1.6.0
py         1.11.0
Pygments   2.19.2
pytest     9.0.2
setuptools 80.9.0
tomli      2.4.0
wheel      0.45.1

• pytest and operating system versions
  MacOS 26.2 (25C56)
  pytest 9.0.2

• minimal example if possible

Files tree:
└── tests
├── common
│   └── test_common.py
├── desktop
│   └── test_desktop.py
└── mobile
└── mobile_test.py

# test_common.py:
def test_case_common():
    pass

# test_desktop.py:
def test_case_1():
    pass

# test_mobile.py:
def test_case_2():
    pass

Outputs:

/check-pytest [5]> pytest -k '/common'
============================= test session starts =============================
platform darwin -- Python 3.12.4, pytest-9.0.2, pluggy-1.6.0
rootdir: /check-pytest
collected 3 items / 3 deselected / 0 selected         
============================= 3 deselected in 0.01s =============================

Start by tracing pytest's -k expression handling and how directory paths are represented during test collection. Reproduce the issue with the provided tests/common, tests/desktop, and tests/mobile layout using pytest -k '/common'. Done means the common directory's test is selected while unrelated tests remain deselected.