Remove pre_approved_agent_privileges from FoundationalFlow

What does this MR do and why?

Ai::Catalog::FoundationalFlow carried two privilege attributes: agent_privileges and pre_approved_agent_privileges. Foundational flows run with no human in the loop, so that split doesn't apply - whatever privileges a flow uses are effectively pre-approved already. Every flow definition had to declare pre_approved_agent_privileges, and the model carried fallback logic that copied it into agent_privileges whenever the latter was left empty.

This MR drops pre_approved_agent_privileges from FoundationalFlow and its per-flow definitions; flows now declare agent_privileges directly. CreateAndStartWorkflowService still populates both agent_privileges and pre_approved_agent_privileges on the Ai::DuoWorkflows::Workflow record it creates - that model also serves human-in-the-loop workflows, so it keeps its own separate pair of attributes.

References

Follow-up of !249586 (merged).

How to set up and validate locally

1. bundle exec rspec ee/spec/models/ai/catalog/foundational_flow_spec.rb
2. bundle exec rspec ee/spec/services/ai/duo_workflows/create_and_start_workflow_service_spec.rb ee/spec/services/ai/catalog/execute_workflow_service_spec.rb
3. bundle exec rubocop on the changed files

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