Handlers never receive widget state because getState() uses wrong property

Problem/Motivation

ConditionalFieldsFormHelper::getState() checks $this->form_state, but afterBuild() sets $this->formState.

Because the undefined property is checked with isset(), the failure is silent. WidgetBase::getWidgetState() is never called and states handlers always receive NULL for $field_info.

No shipped handler currently uses $field_info, so this is not currently user-facing. However, the handler API expects widget state there, and third-party handlers may rely on it.

This was introduced when $form_state was renamed to $formState in #3423942 and this use was missed. It was found while triaging #3499371, but is not the cause of that issue.

Steps to reproduce

1. Create any Conditional Fields dependency and open the entity form.
2. Inspect ConditionalFieldsFormHelper::getState().
3. Confirm WidgetBase::getWidgetState() is never called and $dependee_form_state remains NULL.

Proposed resolution

Replace $this->form_state with $this->formState in getState() so handlers receive the widget state as intended.

Remaining tasks

• Fix the incorrect property name.
• Run the FunctionalJavascript handler coverage to catch any behaviour changes now that handlers receive real widget state.
• Add regression coverage for passing widget state to handlers.
• Consider separately whether the unused $field_info parameter should remain part of the API. Removing it would be a BC break.