Logic error in \Drupal\automatic_updates\Controller\UpdateController::onFinish

Here is the test case we need

1. The test should not start in Maintenance mode but should put into Maintenance module during the update(the default behavior. This would be similar to \Drupal\Tests\automatic_updates\Functional\UpdaterFormTest::testSuccessfulUpdate where $maintenance_mode_on starts as false. Basically no need to call $state->set('system.maintenance_mode', $maintenance_mode_on);

   This will ensure we get inside this IF block

   // Now that the update is done, we can put the site back online if it was
         // previously not in maintenance mode.
         if (!$request->getSession()->remove(BatchProcessor::MAINTENANCE_MODE_SESSION_KEY)) {
   

2. We need a test case where starts off with at least 2 messages, 1 "'Operating in maintenance mode.'" and 1 other one we create.
   That was we will go into the `foreach` loop in the block with an `$messages` array with 1 element, because "Operating in maintenance mode" would have been filtered out.

   $messages = array_filter($messages, function (string $message) {
             return !str_starts_with($message, (string) $this->t('Operating in maintenance mode.'));
           });
           $this->messenger()->deleteByType(MessengerInterface::TYPE_STATUS);
           foreach ($messages as $message) {
             $this->messenger()->addStatus($message);
           }

3. To create the extra message we need a subscriber that just adds a message in `PostApplyEvent`. We could add another test subscriber to automatic_updates_test test module but I think it would be more useful to add the ability to just set messages in \Drupal\package_manager_test_validation\EventSubscriber\TestSubscriber. so we would need a method on that class
   public static function setMessage(string $message, string $message_type, string $event)

   Then we would update \Drupal\package_manager_test_validation\EventSubscriber\TestSubscriber::handleEvent to handle the case where we have message set. See how handleEvent current interacts with the other `set*` methods on that class.

   Then we could set a message in our tests like this
   TestSubscriber1::setMessage("my message", PostApply::class); like we do now for validation results.

4. Then if we set the message correcty "Update Complete" would not show at the end of our test.

   so the end of our test would be

   $assert_session->pageTextContains('Update complete!');
   This line should currently fail, proving the bug

@rahul_ thanks you for making the 2nd MR. I only review https://git.drupalcode.org/project/automatic_updates/-/merge_requests/433

Left some comments there.

@rahul_ I pushed up 2 comment changes. I checked the failures on `3304640-test-fail` branch and they are what we could expect.

This looks good now. will merge if tests still pass/fail as expected with the last commits

Thanks @rahul_! 🎉