Security: Improve admin-ajax.php action sanitization

[Zaidfarooqui01]

Props: mohammadzaid
Fixes: https://core.trac.wordpress.org/ticket/64489

Summary

Improve $_REQUEST['action'] sanitization in admin-ajax.php.

Changes

• Added isset() + is_scalar() checks
• Applied sanitize_key( wp_unslash( $_REQUEST['action'] ) )
• Preserved existing wp_die() guard

Testing

• npm run lint:php ✅
• npm run test:php ✅
• Manual admin AJAX testing ✅

Trac: https://core.trac.wordpress.org/ticket/64489

[github-actions]

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[github-actions]

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[Zaidfarooqui01]

@github-actions thanks!
✅ Linked farooquizaid92@gmail.com to my WordPress.org profile (mohammadzaid)
✅ Added mohammadzaid@git.wordpress.org to GitHub emails