Fix fixture display commands discarding non-zero exit statuses

[lpyu001]

Preserve the exit status returned by showfixtures() and show_fixtures_per_test() instead of unconditionally returning 0 from pytest_cmdline_main().

Collection errors require additional handling because Session.perform_collect() records them in session.testsfailed without raising. Extracted the existing collection-error check from pytest_runtestloop() into _raise_on_collection_errors() and reused it for the fixture display commands. With --continue-on-collection-errors, the commands now return ExitCode.TESTS_FAILED when collection errors occurred.

Added regression tests for both --fixtures and --fixtures-per-test. Pre-fix:

$ python -m pytest --fixtures test_bad.py
...
ERROR test_bad.py
=============================== 1 error in 0.09s ===============================
exit=0

After the fix, collection errors return ExitCode.INTERRUPTED (2), or ExitCode.TESTS_FAILED (1) with --continue-on-collection-errors. Exit statuses produced by internal errors and pytest.exit() are also preserved instead of being replaced with 0.

Closes #15047.

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[bluetech]

Thanks, I made some tweaks but it should be good for pytest 10.0.