Migrate navigator.clipboard in link bubble menu to copyToClipboard

What does this MR do and why?

The link bubble menu copy button called navigator.clipboard.writeText directly, which fails in non-secure contexts. This follows the work in !211222 (merged), which blocks direct use of navigator.clipboard. The component now uses the shared helper copyToClipboard from ~/lib/utils/copy_to_clipboard and the matching eslint-disable comment is removed. The spec now mocks copyToClipboard instead of spying on navigator.clipboard.

Closes #579482.

References

Part of epic &19827.

Screenshots or screen recordings

Not applicable — no visual change.

How to set up and validate locally

1. Run yarn jest spec/frontend/content_editor/components/bubble_menus/link_bubble_menu_spec.js.
2. In GDK, open a project wiki page (or an issue description) and switch to the rich text editor.
3. Add a link, for example [GitLab](https://gitlab.com), and select the link text to open the link bubble menu.
4. Select Copy link URL and paste into another field. The full URL is pasted.
5. Upload a file (for example a PDF) in the same editor, select its link, and select Copy link URL. The pasted value is the full absolute URL, not the relative /uploads/... path.
6. Optional, non-secure context: open GDK over plain HTTP on an address other than localhost (for example http://<your-LAN-IP>:3000). There navigator.clipboard is undefined, so before this change the button threw a TypeError and nothing was copied. With this change, copyToClipboard falls back to document.execCommand('copy') and the URL is copied.

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This contribution was prepared with the help of an AI assistant. I reviewed the changes and take responsibility for them.