CKEditor Validation Accessibility [#3614292]

Problem/Motivation

CKEditor form validation errors are displayed as alerts ([role=alert]) but do not set aria-invalid="true" on the actual input fields, preventing screen readers from properly announcing which field has errors.

AI was used in the creation of this issue.

Steps to reproduce

Key Problems:

When CKEditor plugins have validation errors (e.g., "The following tag(s) are already supported...")
Errors appear as text alerts instead of marking the field as invalid
Screen readers can't identify which field needs correction
Form is not properly announced to assistive technology

Proposed resolution

Modify CKEditor validation JavaScript to set aria-invalid="true" on fields with validation errors
Ensure validation state is cleared when fields are corrected
Test that screen readers can properly announce invalid field state
Verify integration with the existing form label accessibility fix

Files to modify:
- core/modules/ckeditor5/src/ - CKEditor configuration and validation JavaScript
- core/modules/ckeditor5/tests/src/FunctionalJavascript/ - Add tests for aria-invalid behavior

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet

Issue fork drupal-3614292

Show commands

Start within a Git clone of the project using the version control instructions.

Add & fetch this issue fork’s repository

Or, if you do not have SSH keys set up on git.drupalcode.org:

Add & fetch this issue fork’s repository

3614292-ckeditor-validation-accessibility changes, plain diff MR !16506
Check out this branch for the first time

Check out existing branch, if you already have it locally