Migrate ai-gateway E2E test jobs to run against CNG
With CNG pipeline being a faster alternative to the Omnibus build, and with the Omnibus build not being run in MRs by default. It could be beneficial to run the `ai-gateway`-related tests in the CNG pipeline. Currently the `ai-gateway` job runs tests tagged with `:ai_gateway` against the `latest` ai-gateway in mock response mode.
ai-gateway has a helm chart: https://gitlab.com/gitlab-org/charts/ai-gateway-helm-chart
Note: i notice at the above link `⚠️ This helm chart is meant for self-hosted models users only!`. Although we would be using it in with `AIGW_MOCK_MODEL_RESPONSES=true` so I am not sure if it really matters what model we are using.
#### Jobs to be migrated
* `ai-gateway`
* `ai-gateway-no-add-on`
* `ai-gateway-no-license` \<- needs modification to allow for duo core
* `ai-gateway-no-seat-assigned` \<- needs modification to allow for duo core
#### Benefits
1. ai-gateway tests become blocking in MRs
2. ai-gateway tests run automatically in MRs
3. engineers don't need to trigger omnibus pipeline separately (faster feedback)
4. less non-blocking tests in backports
5. better awareness for engineers for writing e2e tests against that use the ai-gateway
## DRI
@tim_beauchamp
epic