Add agent selection dropdown for agentic chat

What does this MR do and why?

Allows users to select a custom agent to chat with. Agents must be configured to be available for the project.

References

Screenshots or screen recordings

How to set up and validate locally

NOTE: Ensure your ai gateway is up to date.

1. Enable the feature flag: echo "Feature.enable(:global_ai_catalog)" | rails c
2. If needed, add some seed data: Setting up GDK for Workflow Catalog Development
3. Open agentic chat, create a new chat. A list of agents should be shown.
4. Speaking with a different agent should alter the output of the chat.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.