Send only root namespace IDs in feature-enabled-by-namespace-ids header

Niko Belokolodovrequested to merge
duo-edit-20251127-233153 into master

What does this MR do?

Reduces the size of the x-gitlab-feature-enabled-by-namespace-ids header by sending only root namespace IDs instead of all namespace IDs.

The change is behind a FF as an extra precaution.

DB query explain: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/46326/commands/141380

Why?

Large numbers of namespace IDs cause gRPC metadata to exceed the 8KB soft limit, triggering ResourceExhausted errors.

Closes #579983

Edited by Niko Belokolodov

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