feat: Open assistant-generated links in a browser

[dcalhoun]

Fixes https://github.com/Automattic/dotcom-forge/issues/7772.

Proposed Changes

Enable users to navigate with links included in assistant responses,
opening the URL in the user's default browser.

Testing Instructions

1. Navigate to the Assistant tab.
2. Request documentation on a subject; e.g., "How do I install a plugin?"
3. Verify clicking links within the assistant response open the link destination
   in the default browser.

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[dcalhoun]

The proposed changes do not include automated tests as we currently mock react-markdown due to the fact that it is an ESM-only library, for which Jest's support is experimental and unstable. Until we configure Jest for ESM and remove the mock, testing the proposed changes is only possible in the form of asserting props passed to react-markdown and/or unit testing Message and Anchor independently, which does not seem all that valuable.

[dcalhoun]

My understanding is the failing Windows E2E tests are unrelated to these changes and currently under investigation (p1718622040527389-slack-C06DRMD6VPZ).

[fluiddot]

My understanding is the failing Windows E2E tests are unrelated to these changes and currently under investigation (p1718622040527389-slack-C06DRMD6VPZ).

Yes, in fact, we've disabled temporarily E2E tests on Windows until it's solved. Sorry for the inconvenience 🙇 .

[sejas]

Nice work! I tested it and I confirm the links open the default browser.

I was thinking if we should check that href starts with http to avoid opening other protocols like mailto. But I guess there is no harm in opening other protocols too.

open-links.mp4