Backport of 'Fix SyncPolicyWorker timeout on linked root namespaces' to 18.10

What does this MR do and why?

Backport of !234994 (merged) to 18-10-stable-ee.

Fixes Security::OrchestrationPolicyConfiguration#linked_source_root_namespaces exceeding the PostgreSQL statement timeout on management projects with a very large number of linked configurations (~19K).

The Ruby-side loop with includes(:project, :namespace) and an N+1 root_ancestor lookup is replaced with a single SQL query anchored on security_orchestration_policy_configurations that resolves each config to its root namespace via traversal_ids[1]. The DB CHECK constraint cop_configs_project_or_namespace_existence guarantees exactly one of project_id / namespace_id is set per row, so COALESCE over the two source paths is safe.

Cherry-picked from merge commit 94d6b78ea5f644927341cb24b1084f3e418272cf with git cherry-pick -m 1.

References

• Original MR: !234994 (merged)
• Issue: #599018 (closed)

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