[6.x] Make PreventsSavingStacheItemsToDisk work with Pest PHP

[aerni]

This PR fixes the PreventsSavingStacheItemsToDisk trait with Pest PHP. The fakeStacheDirectory path is incorrectly calculated, causing test fixtures to be saved to the wrong location.

Root Cause

In AddonTestCase::setUp(), the current code uses reflection on $this to determine the test file path:

$reflection = new ReflectionClass($this);
$this->fakeStacheDirectory = Str::before(dirname($reflection->getFileName()), DIRECTORY_SEPARATOR.'tests').'/tests/__fixtures__/dev-null';

PHPUnit

In PHPUnit tests, $this refers to the actual test class (e.g., MyAddonTest), so $reflection->getFileName() correctly returns the test file path like /path/to/addon/tests/Feature/MyAddonTest.php.

Pest PHP

In Pest PHP tests, $this refers to a dynamically generated class created by Pest internally. As a result, $reflection->getFileName() returns a path inside vendor/pestphp/pest/src/Factories/, causing the fakeStacheDirectory to be set to an incorrect location like vendor/pestphp/pest/src/Factories/tests/__fixtures__/dev-null.

Solution

Use reflection on $this->addonServiceProvider instead of $this. This provides a reliable reference point regardless of whether Pest or PHPUnit is being used, and is consistent with the existing approach in getEnvironmentSetUp():

protected function setUp(): void
    // ...
    $reflector = new ReflectionClass($this->addonServiceProvider);
    $this->fakeStacheDirectory = dirname($reflector->getFileName()).'/../tests/__fixtures__/dev-null';
    // ...
}

protected function getEnvironmentSetUp($app)
{
    // ...
    $reflector = new ReflectionClass($this->addonServiceProvider);
    $directory = dirname($reflector->getFileName());
    // ...
    $app['config']->set('statamic.stache.stores.collections.directory', $directory.'/../tests/__fixtures__/content/collections');
    // ...
}