Fix initialize_admin AJAX calls

[n7studios]

Summary

Fixes this reported issue by ensuring that initialize_admin() is not run during admin-ajax.php requests by explicitly bailing when wp_doing_ajax() is true.

admin-ajax.php runs in an admin context (is_admin() === true) even when triggered from the frontend, which previously caused admin-only classes (including setup wizard importers) to load and execute expensive database queries on frontend AJAX requests.

With #1012 and #1013, the Kit Plugin no longer uses admin-ajax.php (using the REST API instead); this change is safe and prevents unnecessary performance issues on large sites.

Testing

Existing tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)