hotfix: Fix agent-not-found errors: only route refinement to registered agents

[gsarchioni]

Problem

When an agent tried to bounce an idea off another agent, the runtime could log:
ERROR:autogen_core:Agent agent2 not found, failed to deliver message.

find_recipient() was choosing from every agent*.py file on disk. With many concurrent requests, some agents (e.g. agent20) are registered and run before others (e.g. agent2). Picking agent2 before it was registered caused the delivery failure.

Solution

• Track registered agents: Introduced REGISTERED_AGENTS and register_agent() in messages.py. The Creator calls register_agent(agent_name) after each successful registration with the runtime.
• Route only to registered agents: find_recipient() now selects only from REGISTERED_AGENTS instead of from all agent*.py files.
• Exclude self and handle no recipient: find_recipient(exclude=...) excludes the current agent so we don’t send to ourselves. It returns None when there is no valid recipient; the agent then skips the bounce and returns its idea as-is.
• Cleanup: Removed the unused (and broken) from agent import Agent in world.py; the module only defines AgentCreator.

Files changed

• messages.py – REGISTERED_AGENTS, register_agent(), find_recipient(exclude) with registered-only selection
• creator.py – call messages.register_agent(agent_name) after registering an agent
• agent.py – use find_recipient(exclude=self.id.type) and only bounce when recipient is not None
• world.py – remove invalid Agent import

[gsarchioni]

Hi @ed-donner - please review this PR. I noticed an issue with the autogen final project when an agent*.py created but have not been registered to the runtime before another agent calls it.