Widget fails in nested forms (paragraphs): direct getUserInput() access doesn't work with subforms

Problem/Motivation

The Entity Reference Filterable Autocomplete widget only works when used in main node forms but fails when used inside paragraph subforms or other nested form contexts. The issue occurs because the widget directly accesses $form_state->getUserInput()[$fieldName][$delta]['bundle'] which assumes the field is at the top level of the form structure.

Steps to reproduce

1. Add an entity reference field with the "Filterable Autocomplete" widget to a paragraph type
2. Add this paragraph to a node form
3. Try to use the bundle filter dropdown - it won't work properly
4. The AJAX callback also fails to update the autocomplete field

Root cause

In EntityReferenceFilterableAutocompleteWidget.php:

- Line 80-81: Direct array access $input[$fieldName][$delta]['bundle'] only works for top-level forms
- Line 168-169: AJAX callback assumes $form[$fieldName]['widget'][$delta]['target_id'] structure

In nested forms like paragraphs, the actual structure is:

  $input['field_content'][0]['subform'][$fieldName][$delta]['bundle']
  $form['field_content']['widget'][0]['subform'][$fieldName]['widget'][$delta]['target_id']

Proposed resolution

Use Drupal's NestedArray::getValue() utility with proper parent arrays to navigate nested form structures, similar to how core modules like Paragraphs handle this:

1. For form element access: Use $element['#field_parents'] combined with field path
2. For AJAX callback: Use $triggeringElement['#array_parents'] with NestedArray::getValue()