E2E test: relax assertion for duo chat spec

What does this MR do and why?

Relaxes assertion in duo chat spec, as the chat doesn't always respond with GitLab Duo Chat. It's been updated to just check for GitLab

Example failure:

Reproduction steps

Test only runs in staging/prod

• QA_GITLAB_URL=https://staging.gitlab.com bundle exec rspec qa/specs/features/ee/browser_ui/3_create/web_ide/duo_chat_in_web_ide_spec.rb
• QA_GITLAB_URL=https://staging.gitlab.com bundle exec rspec qa/specs/features/ee/browser_ui/16_ai_powered/duo_chat/duo_chat_spec.rb