E2E: Dequarantine Self-managed code_suggestions_spec

What does this MR do and why?

Related QA failure issue: #436465 (closed)

This MR dequarantines the orchestrated self-managed context in ee/api/14_model_ops/code_suggestions_spec.rb.

This also includes removing the self-managed contexts for code generation and streaming, as the AI gateway's fake models do not yet support this. These will be added again in https://gitlab.com/gitlab-org/quality/quality-engineering/team-tasks/-/issues/2301+.

Important: The following MRs must be merged first before this MR can be merged:

• gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!567 (merged)
• gitlab-qa!1251 (merged)
• gitlab-qa!1261 (merged)
• gitlab-qa version bump (gitlab-qa!1264 (merged))
• GitLab dependency gitlab-qa version bump (!142671 (merged))

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.

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Check out this branch locally
2. Export the following env vars:

export GITLAB_LICENSE_MODE=test
export QA_EE_ACTIVATION_CODE=<value from 1Password>
export GITLAB_QA_ADMIN_ACCESS_TOKEN=<value from https://gitlab.com/gitlab-org/gitlab-qa/-/blob/master/support/data/admin_access_token_seed.rb#L7>
export QA_SKIP_PULL=true

1. Build the QA image with

docker build -t gitlab/gitlab-ee-qa:your-custom-tag --file ./qa/Dockerfile ./

1. Tag the current nightly build for the corresponding GitLab image

docker tag gitlab/gitlab-ee:nightly gitlab/gitlab-ee:your-custom-tag

1. Run the following from the qa directory:

gitlab-qa Test::Integration::AiGateway gitlab/gitlab-ee:your-custom-tag