Fix flaky admin impersonation feature spec

What does this MR do and why?

Fixes the flaky examples in spec/features/admin/users/admin_impersonates_user_spec.rb. Full investigation summary https://gitlab.com/gitlab-org/quality/test-failure-issues/-/work_items/43497#note_3620462584

The race

SidebarsHelper reads the merge request counts with cached_only: true, which returns nil on a cache miss rather than running an expensive query. In test Rails.cache is a :null_store, so it always misses and user_counts.vue fires GET /api/v4/user_counts on every page load.

Impersonating calls warden.set_user, which sets renew: true. Rack deletes the old session from Redis and mints a new session ID. A user counts request still in flight carries the old cookie, misses, is handed a fresh anonymous session, and its Set-Cookie overwrites the valid one. The browser is then unauthenticated and DELETE /admin/impersonation fails CSRF verification.

The fix

Warm the count caches in a before hook so the counts ship in the page payload and the fetch is never made. This gives the test the warm-cache behaviour production normally has. COUNT_CACHE_VALIDITY_PERIOD is 24 hours, so on a real instance most page loads never make this request. The :null_store in test is what makes it fire every single time.

A guard asserts the caches are warm, so if cached_only: or the cache keys change, this fails with a clear message rather than quietly returning to flakiness.

References

• https://gitlab.com/gitlab-org/quality/test-failure-issues/-/work_items/43497

How to set up and validate locally

1. Green pipeline 🟢
2. Run the spec locally a few times and confirm there are no flaky failures

MR acceptance checklist

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