Allow agents to call agents

### What would you like to be added?

Currently, agents can not invoke other agents or themselves. It should be allowed, ideally both by specifying the agent name in the tool list (which seems like it would work if it passed validation, but it doesn't, it complains about an invalid tool name), and in some way to allow calling all agents.

### Why is this needed?

This can often be quite useful for the same reason that using agents in the first place is useful, especially context isolation.

### Additional context

_No response_