Improve the test coverage for the fpm-server-requirement-status endpoint.

[techanvil]

Feature Description

The initial implementation of the test coverage for the fpm-server-requirement-status (see #9632) involved mocking the is_endpoint_healthy() protected method of the REST_First_Party_Mode_Controller, due to difficulty testing the code contained within the method.

Notably, although it's possible to mock file_get_contents(), it doesn't appear possible to mock the $http_response_header that gets set locally in the file_get_contents() caller's scope with off the shelf tooling.

site-kit-wp/tests/phpunit/integration/Core/Tags/First_Party_Mode/REST_First_Party_Mode_ControllerTest.php

Lines 235 to 241 in 54f900c

	// Here we mock the `is_endpoint_healthy()` method of the controller. This is necessary because, although we could
	// mock `file_get_contents()`, it's not possible to mock the `$http_response_header` variable used within the scope
	// of the `is_endpoint_healthy()` method. The rest of the controller's behaviour remains unmocked.
	$mock_controller = $this->getMockBuilder( REST_First_Party_Mode_Controller::class )
	->setConstructorArgs( array( $this->settings ) )
	->onlyMethods( array( 'is_endpoint_healthy' ) )
	->getMock();

We should improve the test coverage to ensure the code contained in is_endpoint_healthy() is covered. Possible angles include:

• Refactoring REST_First_Party_Mode_Controller so we pass in say a Guzzle client instance, or our own wrapper to file_get_contents(), and use that rather than calling file_get_contents() directly, allowing us to pass in a mock client during testing and avoid mocking is_endpoint_healthy().
• Adding E2E test coverage for the endpoint. This comes with its own challenges, notably mocking the health check to https://g-1234.fps.goog/mpath/healthy to avoid calling the real Google service from our E2E tests. It looks possible, but would involve some structural changes to our E2E test environment; this was explored during the implementation of 9632, see Add fpm-server-requirement-status E2E test (draft). #9707. A more practical approach requiring less structural changes would be to provide filters for the health check URLs so we could override them during testing. However, the REST_First_Party_Mode_Controller described above is probably the best way to proceed here.

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Implementation Brief

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