fix(security): no direct HTTP access to plugin PHP files (audit L2, cheap part)

Every file under src/ exits outside WordPress (defined('ABSPATH') ||
exit; after the use block, so a direct request no longer prints a fatal
error with the server path); divi-convert-fixture.php gets the WP_CLI
guard the other integration scripts have. A unit test keeps both true;
PSR1 side-effects warning excluded for the guard.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7

src/Integrations/Divi/Pages/WpMediaResolver.php

@@ -6,6 +6,8 @@ namespace LIT\MCP\Integrations\Divi\Pages;
 
 use LIT\MCP\Integrations\Divi\Compose\MediaResolver;
 
+defined('ABSPATH') || exit;
+
 /**
  * Media library images for short-form fields (image_id, portrait_id, image_ids): only images
  * the current user may read. URL and alt text are cleaned by ShortformWriter like client input.