[Agentic Search] Support agent deletion

[ohltyler]

We allow CRU, but missing D for agent interactions on the UI.

We can allow deletion, but should also ensure that users are aware that agent deletion could affect their cluster in other ways, since we allow bringing in existing agents that may be used in other contexts.

The general flow should be as follows:

1. On created/existing workflows, have a button for deletion.
2. When clicking 'Delete', show a confirmation modal. There should be informational text that warns the user to make sure the agent isn't being used anywhere else, and deletion is permanent.
3. Deletion will delete the agent, handle errors, and if successful, set the selected agent to be empty (empty state)