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/Uninstaller.php

@@ -4,6 +4,8 @@ declare(strict_types=1);
 
 namespace LIT\MCP;
 
+defined('ABSPATH') || exit;
+
 /**
  * Removes WP-LIT-MCP's own data (uninstall.php, Task 12): options, the endpoint-check
  * transient, the log-cleanup cron hook and the change log table – on every site of a