Accept AI Catalog agent references in custom flow schema

What does this MR do and why?

First step of #628153 (closed): custom flows will be able to use AI Catalog agents as sub-agents through an include section in the flow YAML. This MR makes Rails accept that shape and gates it behind a feature flag. Nothing resolves the references yet.

• flow_v2.json gets a top-level include list. Each entry names a catalog item: source (only ai-catalog), item_type (only agent), item_id (a string of digits) and version (an exact x.y.z pin). item_id has to be a string because the Duo Workflow Service reads it as one and does not convert a YAML integer.
• A subagents entry in an AgentComponent is now either a {name} object, as before, or the same reference shape.
• Both save services return an error when the definition has an include key and the ai_catalog_flow_agent_references flag is off. The schema file is static JSON, so the flag check is in the services.

The Duo Workflow Service already accepts this shape at save-time validation (ai-assist#2833). It also enforces that a subagent reference repeats an entry in the top-level include list exactly, so the schema being looser here is safe: a definition that references an agent only from subagents fails the DWS call in the save services.

Planned follow-ups from the issue: a resolver for each reference (MR 2), the save-time permission and visibility checks (MR 3), the dependency rows (MR 4, needs #628830 (closed)), and the documentation (MR 5).

References

• Issue: #628153 (closed) (this is MR 1 of the plan in the description)
• Parent epic: &23526
• Reference limits for MR 3: !256851 (merged)
• DWS side: gitlab-org/modelops/applied-ml/code-suggestions/ai-assist#2833 (closed)

Screenshots or screen recordings

No UI change.

How to set up and validate locally

The specs cover the schema and both services:

bundle exec rspec ee/spec/models/ai/catalog/item_version_spec.rb \
  ee/spec/services/ai/catalog/flows/create_service_spec.rb \
  ee/spec/services/ai/catalog/flows/update_service_spec.rb

256 examples, 0 failures.

To see the flag gate from a console: create a flow whose definition has an include entry with Feature.disable(:ai_catalog_flow_agent_references), and the save returns "Referencing AI Catalog items with include is not available".

MR acceptance checklist

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