chore(api): extend core fraction validation error msg

[hardcoretime]

Why do we need it, and what problem does it solve?

The list of possible core fraction values has been added to the validation error message. This change is required to improve user experience.

Checklist

• The code is covered by unit tests.
• e2e tests passed.
• Documentation updated according to the changes.
• Changes were tested in the Kubernetes cluster manually.

Changelog entries

section: api
type: chore
summary: "The core fraction validation error message has been extended to include possible values."
impact_level: low