Tests: Add unit tests for wp_ajax_get_permalink()

[pbearne]

This PR adds unit tests for the wp_ajax_get_permalink() function located in src/wp-admin/includes/ajax-actions.php.

The tests cover:

• Successful retrieval of a post permalink.
• Proper handling of invalid nonces (getpermalink nonce).
• Behavior when the post_id parameter is missing.

Trac ticket: https://core.trac.wordpress.org/ticket/65252

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• Used for: Code analysis, test implementation, and workflow management.

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