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/FieldMapperRegistry.php

@@ -0,0 +1,65 @@
+<?php
+declare(strict_types=1);
+
+namespace ROITheme\Admin\Shared\Infrastructure\FieldMapping;
+
+use ROITheme\Admin\Shared\Domain\Contracts\FieldMapperInterface;
+
+/**
+ * Registro central de Field Mappers
+ *
+ * RESPONSABILIDAD:
+ * - Registrar mappers de cada modulo
+ * - Resolver mapper por nombre de componente
+ *
+ * PRINCIPIOS:
+ * - OCP: Nuevos mappers se registran sin modificar esta clase
+ * - SRP: Solo gestiona el registro, no contiene mapeos
+ */
+final class FieldMapperRegistry
+{
+    /** @var array<string, FieldMapperInterface> */
+    private array $mappers = [];
+
+    /**
+     * Registra un mapper
+     */
+    public function register(FieldMapperInterface $mapper): void
+    {
+        $this->mappers[$mapper->getComponentName()] = $mapper;
+    }
+
+    /**
+     * Obtiene un mapper por nombre de componente
+     *
+     * @throws \InvalidArgumentException Si no existe mapper para el componente
+     */
+    public function getMapper(string $componentName): FieldMapperInterface
+    {
+        if (!isset($this->mappers[$componentName])) {
+            throw new \InvalidArgumentException(
+                "No field mapper registered for component: {$componentName}"
+            );
+        }
+
+        return $this->mappers[$componentName];
+    }
+
+    /**
+     * Verifica si existe mapper para un componente
+     */
+    public function hasMapper(string $componentName): bool
+    {
+        return isset($this->mappers[$componentName]);
+    }
+
+    /**
+     * Obtiene todos los mappers registrados
+     *
+     * @return array<string, FieldMapperInterface>
+     */
+    public function getAllMappers(): array
+    {
+        return $this->mappers;
+    }
+}