Restore falsy attribute values on unpatch

[kwy404]

What:

Unpatching now restores attributes whose original value was falsy.

Why:

The unpatchers in testslide/core/patch.py decided whether to restore or delete based on the truthiness of the saved value. Patching something like SomeClass.count = 0 or a module level FLAG = False with patch_attribute() deleted the attribute when the test finished, which then breaks every later access to it.

How:

Restore when restore is set (which patch_attribute() already computes from whether the attribute existed), keeping the existing value check so the mock_callable() callers behave as before.

Risks:

Low. The only behavior change is for attributes that existed with a falsy value.

Checklist:

• Added tests, if you've added code that should be tested
• Updated the documentation, if you've changed APIs (N/A)
• Ensured the test suite passes
• Made sure your code lints
• Completed the Contributor License Agreement ("CLA")

I ran the *_testslide.py suites and the unittest suites locally on Python 3.12, plus ruff, flake8 and mypy. The new example in tests/patch_attribute_testslide.py fails before the change. mock_constructor_testslide.py has one failure on 3.12 (can call class methods) and cli_unittest needs termios, both the same on main.

[meta-codesync]

@oxo42 has imported this pull request. If you are a Meta employee, you can view this in D122127123.