Fix documentation tool problem

What does this MR do and why?

With lot of changes concentrated on the GitLab Duo chat there were some changes that also touched legacy documentation chat and caused regression. It is fixed in this MR.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Setup chat according to: https://docs.gitlab.com/ee/development/ai_features.html#set-up-gitlab-duo-chat
2. 1. In rails console enable

   Feature.enable(:ai_chat_prompt_alternative)

3. In rails console enable the experiment fully

   Feature.enable(:gitlab_duo)

4. Ask documentation-related question and observe results.
5. In rails console disable the experiment fully

Feature.disable(:gitlab_duo)

1. Ask documentation-related question and observe results.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.