Adds isValid state & feedback to dropdown; Removes projectId auto-select

What does this MR do and why?

This MR ensures that when a user attempts to add a public agent to a project, the agent's existing project association is not pre-selected. It also uses the standard red border UI affect on the project dropdown element to highlight what the user needs to do next.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Switch to the 569660-aryer-replace-input-with-dropdown branch (I named this incorrectly, I apologize), install dependencies
2. Navigate to http://gdk.test:3000/explore/ai-catalog/agents/
3. Click the kebab icon on a public project, and click "+ Add to project"

MR acceptance checklist

• Project dropdown should not be populated when the "+ Add to project" button is clicked from a public project
• Project dropdown selector should show the red border indicating an invalid state while there is no project selected

Related to #569660 (closed)