`call_on_fail()` no longer registers globally since 2.9.1

[Bastian-Krause]

Since ba3fdc9 ("corner case fix") introduced in 2.9.1, call_on_fail() no longer registers session global, but tied to the test phase. This breaks the example in https://github.com/okken/pytest-check#logging-to-a-file as well as examples/logging_to_a_file/.

On 2.8.0:

$ pytest -q examples/logging_to_a_file/ > /dev/null
$ cat session.log
--- 2026-08-13 11:58:03.366 ---
---------
Starting test run
---------
--- 2026-08-13 11:58:03.373 ---
FAILURE: check 1 == 2
examples/logging_to_a_file/test_file_logging.py:4 in test_one() -> check.equal(1, 2)

--- 2026-08-13 11:58:03.375 ---
FAILURE: check 5 == 6
examples/logging_to_a_file/test_file_logging.py:7 in test_two() -> check.equal(5, 6)

On 2.9.1:

$ pytest -q examples/logging_to_a_file/ > /dev/null
$ cat session.log
--- 2026-08-13 11:58:25.393 ---
---------
Starting test run
---------