E2E test: add code completion in WebIDE e2e test

What does this MR do and why?

Partially resolves #434110 (closed)

This MR adds a new test case to exercise code completion in the Web IDE. The existing test exercises code generation, so it has been explicitly identified as such.

I have tested this locally against staging and production. I have created a placeholder test cases and linked it also.

How to set up and validate locally

This test only runs against live environments. To run against staging:

QA_LOG_LEVEL=debug 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/code_suggestions_in_web_ide_spec.rb

I also tested the new prompt in production to ensure the same/similar prompt was being returned.