feat: add a get_wiki_page tool

What does this merge request do and why?

Create a tool to allow reading wiki pages and its comments using Duo. This is meant to be a short term solution while we work on a longer term way to add wikis to DAP using MCP tools when the foundation for that is in place.

• Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/561715
• Slack convo: https://gitlab.slack.com/archives/C09JKA0UX0C/p1761770515462019
• Related MR in the monolith: gitlab-org/gitlab!211298

How to set up and validate locally

1. Ensure GDK is setup locally with AI gateway enabled
2. Check out branch himkp-wiki-tool in gdk/gitlab
3. Check out branch himkp-wiki-tool in gdk/gitlab-ai-gateway
4. gdk reconfigure && gdk restart
5. Create a new agent, and add the get_wiki_page tool to it. Add the agent to a project.
6. Go to a wiki page (or create one) that has some comments, and ask it questions about the page, and its comments.

Wiki page and comments in one tool
CleanShot 2025-11-06 at 16.10.18.mp4

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.
• If this change requires executor implementation: verified that issues/MRs exist for both Go executor and Node executor or confirmed that changes are backward-compatible and don't break existing executor functionality.