E2E test: generalise duo chat assertion

What does this MR do and why?

Modifies the assertion for Duo Chat e2e tests.

Addressing issues in https://gitlab.com/gitlab-org/gitlab/-/issues/509129

Rather than checking for a specific string in the Duo Chat response, we will check for the following:

• any response
• feedback message
• absence of a duo chat error

Since the assertion is the same for gitlab.com and SM, we can remove the logic to try and figure out which environment we are on.

We may consider combining the gitlab.com and SM in a follow up.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

It's an orchestrated test, so results can be viewed in the omnibus pipeline. e.g. https://gitlab.com/gitlab-org/gitlab/-/jobs/8682846571#L420

Against staging:

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