Fix Translate Text Explorer form functionality

Problem/Motivation

When using the Translate Text Explorer, it always results in an exception:

An error occurred
The following error occurred while processing your request:

Form errors cannot be set after form validation has finished.

Of the following exception type:

LogicException

The reason seem to be the inconsistent use of a prefix in the form code.

Steps to reproduce (required for bugs, but not feature requests)

Have an translate text type of a provider (like ai_provider_deepl) enabled, and go to /admin/config/ai/explorers/translation_generator, trying to submit the form.

Proposed resolution

Fix the prefix in a function call.