Visually indicate AI generated inserted text

Is your feature request related to a problem? Please describe.
As a possible follow-up to https://github.com/nextcloud/text/issues/8782, when we directly insert AI-generated content, it’s important to make this visible.

We can do this via infobox-like containers, or also nicer.

Describe the solution you'd like
iA Writer has a very nice way of showing AI generated text inline, sort of like a collaborator color highlighting. Scroll a bit down on https://ia.net/writer – could be interesting to explore

https://github.com/user-attachments/assets/f9e0d5df-8578-4876-8b3f-fc10c830d6ce

We can use the same color fade we use for Assistant text:

Describe alternatives you've considered
Simple infoboxes we already support. We could introduce a new Assistant-styled infobox that you can’t manually use? (Or you could, to highlight content you pasted from external AI tools?)

Quick mockup (of course the left border should have the proper gradient):

Additional context
They have a blog post about this with more design thoughts on it: https://ia.net/topics/ia-writer-7
It’s from 2023 so a bit dated, and they initially dimmed the text you paste from AI tools.

One relevant info from the post is that they published a Markdown spec for it: https://github.com/iainc/Markdown-Annotations

As discussed in Office/Productivity planning.

No implementation files or tests are named. Start by reviewing the existing Assistant text styling and supported infoboxes, then compare the iA Writer examples and Markdown Annotations reference; done means agreeing on and implementing a clear visual treatment for directly inserted AI-generated text.