refactor(admin): Migrate AdminAjaxHandler to Clean Architecture

- Move AdminAjaxHandler to Admin/Shared/Infrastructure/Api/Wordpress/
- Create FieldMapperInterface for decentralized field mapping
- Create FieldMapperRegistry for module discovery
- Create FieldMapperProvider for auto-registration of 12 mappers
- Add FieldMappers for all components:
  - ContactFormFieldMapper (46 fields)
  - CtaBoxSidebarFieldMapper (32 fields)
  - CtaLetsTalkFieldMapper
  - CtaPostFieldMapper
  - FeaturedImageFieldMapper (15 fields)
  - FooterFieldMapper (31 fields)
  - HeroFieldMapper
  - NavbarFieldMapper
  - RelatedPostFieldMapper (34 fields)
  - SocialShareFieldMapper
  - TableOfContentsFieldMapper
  - TopNotificationBarFieldMapper (17 fields)
- Update functions.php bootstrap with FieldMapperProvider
- AdminAjaxHandler reduced from ~700 to 145 lines
- Follows SRP, OCP, DIP principles

BACKUP BEFORE: Removing CTA A/B Testing legacy system

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Admin/Shared/Infrastructure/FieldMapping/FieldMapperProvider.php

@@ -0,0 +1,68 @@
+<?php
+declare(strict_types=1);
+
+namespace ROITheme\Admin\Shared\Infrastructure\FieldMapping;
+
+use ROITheme\Admin\Shared\Domain\Contracts\FieldMapperInterface;
+
+/**
+ * Provider para auto-registro de Field Mappers
+ *
+ * RESPONSABILIDAD:
+ * - Descubrir automaticamente FieldMappers en cada modulo
+ * - Registrarlos en el FieldMapperRegistry
+ *
+ * BENEFICIO:
+ * - Agregar nuevo componente = crear FieldMapper (sin tocar functions.php)
+ * - Eliminar componente = borrar carpeta (limpieza automatica)
+ */
+final class FieldMapperProvider
+{
+    private const MODULES = [
+        'TopNotificationBar',
+        'Navbar',
+        'CtaLetsTalk',
+        'Hero',
+        'FeaturedImage',
+        'TableOfContents',
+        'CtaBoxSidebar',
+        'SocialShare',
+        'CtaPost',
+        'RelatedPost',
+        'ContactForm',
+        'Footer',
+    ];
+
+    public function __construct(
+        private readonly FieldMapperRegistry $registry
+    ) {}
+
+    /**
+     * Registra todos los FieldMappers disponibles
+     */
+    public function registerAll(): void
+    {
+        foreach (self::MODULES as $module) {
+            $this->registerIfExists($module);
+        }
+    }
+
+    /**
+     * Registra un mapper si existe la clase
+     */
+    private function registerIfExists(string $module): void
+    {
+        $className = sprintf(
+            'ROITheme\\Admin\\%s\\Infrastructure\\FieldMapping\\%sFieldMapper',
+            $module,
+            $module
+        );
+
+        if (class_exists($className)) {
+            $mapper = new $className();
+            if ($mapper instanceof FieldMapperInterface) {
+                $this->registry->register($mapper);
+            }
+        }
+    }
+}