…tests

The Task services no longer set flash messages — that responsibility moved to
the Controller. The writes_success_flash unit tests were removed from the
three TaskTests, leaving the flash behavior untested.

Add three functional tests to ControllerTest:

- confirm_registration_sets_success_flash: creates a PendingOptIn, follows
  the confirm URL, follows the redirect to the edit page, and asserts a
  .flash-success element is rendered.
- edit_registration_sets_success_flash: submits the edit form via the
  crawler and asserts the flash appears in the re-rendered page.
- delete_registration_sets_success_flash: posts to the delete URL, follows
  the redirect to the start page, and asserts the flash appears there.

Two supporting additions were needed:

- DummyRecipientFactory: the confirm flow calls RecipientFactory, which uses
  DetermineAppsSubclassHelper to find a RecipientInterface implementation
  outside the bundle namespace. No such class is declared in the test kernel
  (Dummy\Recipient is excluded because its namespace starts with
  Webfactory\NewsletterRegistrationBundle). DummyRecipientFactory bypasses
  the scan and creates Dummy\Recipient directly, which is the Doctrine-mapped
  entity the test kernel uses. Wired via functional_services.php.
- NewsletterFactory::createMany(2) in the edit test: the newsletters form
  field — and with it the submit button — is only added to the form when at
  least two visible newsletters exist in the database.