E2E tests: add SM duo chat scenario

What does this MR do and why?

Resolves: https://gitlab.com/gitlab-org/gitlab/-/issues/439681

Adds Duo Chat E2E test for a self managed instance against ai-gateway using a fake model. (Relevant discussion gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!695 (comment 1909870556) <- this shows what is displayed by Duo Chat when using a fake.)

Also modifies assertion to just check that any message is returned by Duo Chat. (discussion)

How to set up and validate locally

To test the self-managed version of the test:

CHROME_DISABLE_DEV_SHM=true QA_IMAGE="registry.gitlab.com/gitlab-org/gitlab/gitlab-ee-qa:jmc-duo-chat-self-managed"  DOCKER_DEFAULT_PLATFORM=linux/amd64    GITLAB_LICENSE_MODE=test QA_EE_ACTIVATION_CODE= GITLAB_QA_ACCESS_TOKEN= bundle exec gitlab-qa Test::Integration::AiGateway EE

To test the gitlab.com version:

GITLAB_URL=https://staging.gitlab.com  bundle exec rspec qa/specs/features/ee/browser_ui/16_ai_powered/duo_chat/duo_chat_spec.rb --tag=external_ai_provider