Treat errors in pytest_unconfigure as internal errors

[rafael-g-v]

Closes #9808

Exceptions raised in pytest_unconfigure hooks escaped wrap_session and made pytest exit with code 1 and a raw traceback. Now they are reported as an internal error (exit code 3), the same as exceptions in pytest_configure.

• Wrapped config._ensure_unconfigure() with the same handling already used
  for pytest_sessionfinish.
• Added a test in testing/test_main.py and a changelog entry.

I'm a student; I used an AI assistant to help find the cause and write the change, and I reviewed and tested it myself. Any feedback is welcome!

[rafael-g-v]

I updated 4 existing tests (in test_threadexception.py and test_unraisableexception.py) that expected the old behavior: exit code 1 and the traceback in stderr. With this change, errors raised during config cleanup are reported as an internal error (exit code 3) with INTERNALERROR> lines in stdout.

Some of these tests have a TODO saying the outcome should be a test failure or error. Is exit code 3 acceptable here, or would you prefer a different handling for these cases?

The Read the Docs failure seems unrelated to this PR: the only warning is "py:class reference target not found: pluggy._compat.DistFacade", from the PluginManager.list_plugin_distinfo docstring in the new pluggy 1.7.0 release.

[rafael-g-v]

Hi @nicoddemus, sorry for the ping! When you have some time, could you take a look at this PR? I'd especially like to know if changing the expected exit code in the existing unraisable/thread exception tests is acceptable. Thanks!