E2E test: add web ide duo chat spec (live envs only)

What does this MR do and why?

This MR adds a test to ensure Duo Chat E2E flow working correctly via the web ide. This test is restricted to live environments only. Future iterations will involve getting the test working in the GDK/omnibus (potentially with a stubbed AI gateway).

In this test we expect Duo Chat to be opened from the Web IDE and to respond to "hi" with "Hello!"

Resolves https://gitlab.com/gitlab-org/gitlab/-/issues/439171

How to set up and validate locally

This test will only work in an environment with a working AI gateway. e.g. staging

From the qa directory:

GITLAB_QA_USER_AGENT=<1P> GITLAB_USERNAME=gitlab-qa GITLAB_PASSWORD=<1P> GITLAB_QA_ACCESS_TOKEN=<1P> 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

Checklist

• Confirm the test has a testcase: tag linking to an existing test case in the test case project.
• Follow the end-to-end tests style guide and best practices.
• Use the appropriate RSpec metadata tag(s).
• Most resources will be cleaned up via the general cleanup task. Check that is successful, or ensure resources are cleaned up in the test:
• Verify the tags to ensure it runs on the desired test environments.