E2E test: update duo chat assert

What does this MR do and why?

Resolves https://gitlab.com/gitlab-org/gitlab/-/issues/498171

With gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!1443 (merged) the ai-gateway now returns a different mock response for Duo Chat.

Test failing: https://gitlab.com/gitlab-org/gitlab/-/jobs/8056925511#L631

Test passing: https://gitlab.com/gitlab-org/gitlab/-/jobs/8056935039#L445