Add #required to "OpenAI API Key" field

Problem/Motivation

Field "OpenAI API Key" is required, but does not have client-side validation for required. We just need to add #requiredproperty, which would be useful to have client-side validation itself.

Steps to reproduce

• Go to admin/config/ai/providers/openai
• Save the Form without selecting a key.

Proposed resolution

Add a property

      '#required' => TRUE,

Remaining tasks

Implement the code

User interface changes

API changes

Data model changes