AJAX: Send wp link query responses as JSON

[ArkaPrabhaChowdhury]

Summary\n\n- Use wp_send_json() for wp_ajax_wp_link_ajax() responses.\n- Add regression coverage for the JSON content type and response payload.\n\nTrac: https://core.trac.wordpress.org/ticket/49408\n\n## Testing\n\n- PHPCBF\n- PHPCS\n- PHP syntax checks\n- PHPUnit blocked locally because the Docker MySQL host is unavailable.

[github-actions]

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