The add new carrier example won't obey free shipping rules

### Is there an existing issue for this?

- [X] I have searched the existing issues

### What's missing?

In the guide for `Add a custom shipping carrier` https://developer.adobe.com/commerce/php/tutorials/frontend/custom-checkout/add-shipping-carrier/ it seems the example is missing critical functionality

The new carrier is added properly but it won't obey free shipping rules from cart rules because the collectTotals method is not accounting for that.

A fully working example should be given for a simple method to actually help developers build proper shipping methods.

If you could clarify this point it would help a lot.

Start with the “Add a custom shipping carrier” guide at the linked Adobe Commerce documentation URL, focusing on the example’s collectTotals method and its handling of cart-rule free shipping. Update the example and explanatory text so the carrier demonstrates a working free-shipping-aware method, then verify the guide clearly explains the expected behavior.

Written by the indexing model from the issue text.