Fix potential TypeError from getcwd() in SelectorTestCase

[Copilot]

getcwd() returns string|false, but ContainerFactory expects string — passing false causes a TypeError that silently breaks all selector tests in CI/container environments where the working directory is unavailable.

Changes

• tests/unit/selectors/SelectorTestCase.php: Replace getcwd() with dirname(__DIR__, 3) to derive the project root deterministically at compile time

// Before
$factory = new ContainerFactory(getcwd());

// After
$factory = new ContainerFactory(dirname(__DIR__, 3));

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