E2E test: add test for explaining code via duo chat

What does this MR do and why?

Resolves #460807 (closed)

Adds an E2E test to explain code in a repository. The test highlights a keyword, and clicks the ❓ button. The question button opens duo chat with /explain The test checks that some response is returned.

This test requires an :external_ai_provider to run, so it only runs in live environments.

I also ran the gitlab-qa image from this MR against staging and the test passes: https://ops.gitlab.net/gitlab-org/quality/staging/-/jobs/14589318#L1071

MR acceptance checklist

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Screenshots or screen recordings

dc-explain

How to set up and validate locally

note requires GDK with working duo chat QA_LOG_LEVEL=DEBUG QA_GITLAB_URL=https://gdk.test:3443 GITLAB_QA_PASSWORD_1=x bundle exec rspec qa/specs/features/ee/browser_ui/3_create/repository/duo_chat_explain_code_spec.rb

alternatively can be run against staging: QA_LOG_LEVEL=debug GITLAB_QA_USER_AGENT=x GITLAB_USERNAME=gitlab-qa GITLAB_PASSWORD=x GITLAB_QA_ACCESS_TOKEN=x QA_GITLAB_URL=https://staging.gitlab.com bundle exec rspec qa/specs/features/ee/browser_ui/3_create/repository/duo_chat_explain_code_spec.rb