[Agentic Search] Improve agent form field validation

[ohltyler]

Agent form fields should be a more formal (no pun intended), consistent, and user-friendly experience, such that required fields are highlighted if missing/invalid, and all fields are submitted / validated on create/update button actions.

Implementation-wise, the plugin should have a dynamically-generated, nested formik form as users are configuring.

Currently, the plugin just disables the create/update buttons for certain scenarios, like empty model ID fields (if applicable), or for the QueryPlanningTool. This should be more comprehensive and informative.