Enhancement: AI Assistant "Fetch URL" feature

### Impacted plugin

Jetpack

### What

It would be great if the AI Assistant had a way to fetch specific URLs based on a string

### How

Whenever I'm writing something about things on the internet, I like to make sure that I link to the URL of that thing.

This often means I have to go find that URL, to insert it as a link over the text (thank goodness for the block editors' "paste URL" functionality)

It would be wonderful if I could select the text I want to link, hit the AI Assistant button in the block toolbar, and have the option to search for the URL for that text, which I could then just copy and paste over the linked text.

So for example, if I select "Visual Code Studio" and then hit a button that did something like "Get the URL for Visual Code Studio", it returned the URL as the link for the text I selected.

The issue names the Jetpack AI Assistant, block toolbar, and block editor, but no files or tests. Start by locating the assistant’s block-toolbar entry point; done means selecting text can request and return a specific URL for copying.

Written by the indexing model from the issue text.