Add Flow Creator foundational chat agent

What does this MR do and why?

Adds the Flow Creator to the list of foundational chat agents. This makes the agent selectable in Duo Chat, where it helps users create custom flow YAML for the AI Catalog by describing what they want in plain language.

The agent's flow definition lives in the AI Gateway and is added in ai-assist!6153. This MR is the GitLab side of the pair: without this entry, the chat UI has no way to offer or start the agent.

I tested this locally with both changes in place. The agent appears in the chat agent list, starts a session, and produces flow YAML output.

References

• Primary epic that motivates this change: "Flow Creator" foundational agent (gitlab-org#22644)
• AI Gateway flow definition: feat(flow-registry): add Flow Creator as a foun... (gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!6473 - merged)

How to set up and validate locally

1. In your gitlab-ai-gateway checkout, check out the flow_creator_agent branch from ai-assist!6473
2. Checkout the latest main branch on your AI Gateway
3. Check out this branch in your gitlab checkout.
4. Open Duo Chat in the browser and switch the agent to Flow Creator.
5. Ask it to build a flow, for example: "Create a flow that summarizes an issue when I am assigned to it."
6. Confirm the agent responds and produces flow YAML.

Run the existing spec:

bundle exec rspec ee/spec/lib/ai/foundational_chat_agents_definitions_spec.rb

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