fix(test): reset is_windows() mock state in CIUnitTestCase tearDown

[gr8man]

Description
When mocking the operating system via is_windows(true) or is_windows(false), the internal static variable $mocked retains the set boolean value until is_windows(null) is explicitly invoked with an argument (func_num_args() === 1).

In CommonFunctionsTest::testIsWindowsUsingMock(), line 866 previously called is_windows(); with zero arguments. Because zero arguments were passed, $mocked was not reset and remained false. This resulted in test pollution for any subsequent tests executed in the same PHPUnit process that relied on auto-detection of the OS platform (DIRECTORY_SEPARATOR === '\\').

This PR addresses the issue by:

1. Updating CommonFunctionsTest::testIsWindowsUsingMock() to call is_windows(null) so that it explicitly tests resetting the mock.
2. Adding resetIsWindowsMock() to CIUnitTestCase::$tearDownMethods to automatically reset is_windows(null) after each test run, preventing static state leaks across the test suite.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated (Not applicable - test framework internal fix)
• Conforms to style guide

[michalsn]

Please add a dedicated regression test verifying that CIUnitTestCase::tearDown() resets the is_windows() mock. The updated CommonFunctionsTest resets it explicitly, so it doesn't cover automatic cleanup.

[michalsn]

The existing test implementation could overwrite this. Instead, please just reset it directly at the end of the tearDown() method.

[michalsn]

Do we have to check for this function at this point? I don't believe so.