Update create AI Agent form to used shared form component

What does this MR do and why?

This MR updates the the create AI Agent form to use the shared form component. This will allow us to use the same form component for both create and edit views.

MR acceptance checklist

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

How to set up and validate locally

1. Ensure you have an EE license activated

2. Enable the feature flag

   Feature.enable(:agent_registry)

3. Load /-/ml/agents for any project (for example /flightjs/Flight/-/ml/agents) and click Create agent

4. Ensure the New Agent form loads with inputs for name and prompt.

5. Input an Agent Name and Prompt and click Create agent.

6. Load the list view again at /-/ml/agents and ensure the agent entry has been created.