Restore rel key in loop add-to-cart filter arguments - #66856

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• I have followed the WooCommerce Contributing Guidelines and the WordPress Coding Standards.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have reviewed my code for security best practices.
• Following the above guidelines will result in quick merges and clear and detailed feedback when appropriate.

Changes proposed in this Pull Request:

In #66743 rel was conditionally removed from Product Button. But we should not change established shape of the public woocommerce_loop_add_to_cart_args filter arguments. This restores the rel key with an empty value for permalink anchors in the classic and Product Button PHP paths, while retaining nofollow for direct add-to-cart and external links.

Closes #66855.

Bug introduced in PR #66743.

How to test the changes in this Pull Request:

Using the WooCommerce Testing Instructions Guide, include your detailed testing instructions:

1. Add a callback to woocommerce_loop_add_to_cart_args that appends sponsored to $args['attributes']['rel'] without first checking whether the key exists:

add_filter(
	'woocommerce_loop_add_to_cart_args',
	function ( $args ) {
		$args['attributes']['rel'] .= ' sponsored';
		return $args;
	}
);

3. View a variable product in both a classic Shop archive and a Product Collection. Confirm there is no undefined-array-key warning and the rendered permalink button includes the appended sponsored relationship without nofollow.

Testing that has already taken place:

• PHP syntax checks passed for all four changed PHP files.
• Changed-file PHP lint passed.
• Full committed branch PHP lint passed.
• PHPStan completed with no errors for the two changed production files.
• Targeted PHPUnit was not run locally because this worktree does not have its Composer and Node dependencies installed.

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