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/DiviSaver.php

@@ -15,6 +15,8 @@ use Throwable;
 use WP_Error;
 use WP_Post;
 
+defined('ABSPATH') || exit;
+
 /**
  * Save pipeline of spec §5.6, steps 3–5: restore point and wp_update_post() with wp_slash() (the
  * status stays), Divi 5 flags (DiviFlags), Divi's static CSS cache. A failure after saving is