[Admin/products] Show existing tags when typing new tag

Description

- As a: enterprise user - On page: admin/products - I want to be able to do: see suggestions of existing tags when starting to type a new tag. This would allow users to select an existing tag and avoid typos when adding an existing tag.

The dropdown can show all tags when selected and only those which match the first letters typed once the user starts typing

This behavior already exist inadmin/customersbut only if the tag has been linked with a tag rule :

<img width="230" height="115" alt="Image" src="https://github.com/user-attachments/assets/39b6f63d-ad89-4b3f-a243-86184d5c929b" />

here we want to be able to show that dropdown regardless of tag rules being used or not.

Current behavior on products

No suggestion are shown, which means here if I click save I can end up with 2 different tags:

<img width="1458" height="722" alt="Image" src="https://github.com/user-attachments/assets/82de7acd-6e97-4a1e-b4a0-524b1d844f77" />

Acceptance Criteria & Tests

1. Go to admin/products with variants tags feature toggle ON (should be the default when creating a new enterprise)
2. Add a tag on a first variant
3. See a dropdown with that first tag as suggestion when trying to add a tag on another variant (from same product or a different one)
4. If the new tag is different from ay tags created befor,e the dropdown should not show
5. Typing letters should reduce the suggestion in the dropdown. We should consider first letters, in the same order, not just any letters contained in the word
6. Dropdown content should be ordered alphabetically