Make the shared context more clear about what it is

What does this MR do and why?

Make the shared context more clear about what it is

Also do not allow changing let to alter the context, because it's confusing that the context we included can be altered.

I was quite confused when trying to track what is the reason why we need to set different stubs for !2256 (comment 1407723348)

It turned out that valid_command? is implicitly stubbed and this means in other context we should follow with similar approach or this context should be broken down.

I think renaming in !1515 (merged) makes sense, but then I think the context need to be more clearly described.

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-policies-with-a-dry-run on how to perform dry-runs for new policies.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/doc/reactive/best_practices.md#use-the-sandbox-to-test-new-processors on how to make sure a new processor can be tested.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review