Treat malformed unittest skips as collection errors

[zhangli091011]

Closes #10821.

Summary

Malformed use of an aliased unittest.skip decorator currently raises unittest.SkipTest from unittest's generated skip_wrapper while importing the test module. Pytest classifies every collection-time unittest.SkipTest as a module skip, so this programming error can hide all tests in that module.

This keeps legitimate unittest module skipping intact and changes only SkipTest whose innermost traceback frame is unittest's generated skip_wrapper. Such exceptions are now normal collection errors. Direct module-level raise unittest.SkipTest(...) remains a skipped module, matching unittest.TestLoader.discover, and pytest.skip(..., allow_module_level=True) is unaffected.

This narrower boundary addresses the compatibility concern raised on #10864 and #14415 rather than removing collection-time SkipTest handling wholesale.

Tests

• python -m pytest testing/test_unittest.py testing/test_runner.py testing/test_skipping.py (231 passed, 9 skipped, 1 xfailed)
• tox -e py310 -- testing/test_unittest.py -k "raising_unittest_skiptest_during_collection or unittest_skip_decorator_misuse_is_collection_error" (2 passed)
• python -m pytest testing/test_unittest.py -k "raising_unittest_skiptest_during_collection or unittest_skip_decorator_misuse_is_collection_error" on Python 3.13 (2 passed)
• pre-commit run --all-files --show-diff-on-failure (passed, including Ruff, formatting, mypy, RST and policy hooks)

Current main was reproduced separately: the issue example reports collected 0 items / 1 skipped before this change.

AI assistance

OpenCode assisted with repository inspection, implementation, and validation, and is credited by a Co-authored-by trailer. I reviewed the complete diff and tests, understand the traceback-based compatibility boundary, and take responsibility for this change and reviewer follow-up in accordance with pytest's AI/LLM-Assisted Contributions Policy.

• Includes a regression test.
• Includes a changelog entry.
• Adds the contributor to AUTHORS.
• Allows maintainers to push and squash when merging.
• Credits the AI agent in the commit trailer.

[The-Compiler]

The submitting account owner must review, understand, and take responsibility for this change

This... doesn't exactly convince me that this actually happened?