[#3574404] Enforce non-empty values for required string fields in JSON Editor forms

Summary

Adds client-side validation that rejects empty or whitespace-only values for required string fields in JSON Editor forms. Previously, the required keyword only checked property presence, allowing empty strings to pass silently.

Changes

Custom validator (js/patternkit.jsoneditor.js)

• Validates required string fields produce "Value required" errors when empty
• Idempotent registration via flag to prevent duplicates across attach() cycles

Media field error display (patternkit_media_library/.../media_library.js)

• Overrides showValidationErrors() in DrupalMediaEditor — the parent AbstractEditor has a no-op implementation, so errors were silently ignored

Form submission gates (js/patternkit.jsoneditor.js)

• Non-AJAX: onSubmit() blocks submission and shows inline errors
• AJAX: validateBeforeAjaxSubmit() intercepts submit, resets AJAX state on failure so the button remains usable

AJAX wrapper rework (js/patternkit.jsoneditor.js)

• Replaced hasOwnProperty guards with explicit wrap-once flags
• Resolved stale closure references — wrappers now resolve the active editor via window.patternkitEditor at call time
• Added null-safety for previous handler delegation

Testing

• Open Layout Builder → add Patternkit block with required string field
• Leave field empty → inline "Value required" error appears
• Click save → submission blocked, error visible
• Fill in field → error clears, save succeeds
• Verify media fields also show inline errors when required and empty
• Verify submit button stays enabled after validation failure (AJAX)

Closes #3574404.