``ExceptionInfo.for_later()`` wont add assertion strip text

Using exc_info.exconly(tryshort=True) is meant to remove AssertionError: from the exception message according to (ref)

When 'tryshort' resolves to True, and the exception is an AssertionError, only the actual exception part of the exception representation is returned (so 'AssertionError: ' is removed from the beginning).

However, this is not the case. In addition, the tests do not assert this behaviour:

https://github.com/pytest-dev/pytest/blob/44895289c725ed922889c882b7f1b78b741eba20/testing/code/test_excinfo.py#L332-L339

MWE

cat <<-EOF > excinfo_exconly.py
import pytest

def test():
    with pytest.raises(AssertionError) as exc_info:
        raise AssertionError('message')

    assert exc_info.exconly(tryshort=True) == 'message'
EOF
pytest excinfo_exconly.py

Environment

Platform:              linux; (Linux-5.14.21-150500.55.52-default-x86_64-with-glibc2.31)
Python version:        3.10.13 (main, Sep 05 2023, 11:46:10) [GCC])
Python implementation: CPython
Pytest version:		   8.0.2

Reproduce the MWE with pytest, then inspect testing/code/test_excinfo.py around lines 332-339 and the ExceptionInfo.for_later()/exconly() behavior. Add coverage for an AssertionError with tryshort=True, and confirm the result is "message" without the "AssertionError: " prefix.