Issue #3617864: TemplateForm and TemplateDeleteForm typehint concrete Messenger class, breaking decorated messenger services

Rajendar Pothireddypallyrequested to merge
issue/workbench_email-3617864:3617864-templateform-and-templatedeleteform into 3.x

Typehints MessengerInterface instead of the concrete Messenger class in TemplateForm and TemplateDeleteForm, so decorated/replaced messenger services no longer throw a TypeError.

No behavioural change — both forms only call addMessage() / addStatus(), both declared on MessengerInterface.

See #3617864 for reproduction and precedent (#2852828 in this module, entity_clone #3246560).

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