feat: pydantic models and inter-process messaging

Moved `InternalMessage` into schemas and created a `BeliefMessage`
model. Also added the ability for agents to communicate via ZMQ to
agents on another process.

ref: N25B-316

test/unit/core/test_agent_system.py

@@ -2,6 +2,7 @@
 
 import asyncio
 import logging
+from unittest.mock import AsyncMock
 
 import pytest
 
@@ -39,7 +40,7 @@ async def test_agent_lifecycle():
 
     # Wait for task to finish
     await asyncio.sleep(0.02)
-    assert len(agent._tasks) == 1  # _process_inbox is still running
+    assert len(agent._tasks) == 2  # message handling tasks are running
 
     await agent.stop()
     assert agent._running is False
@@ -51,14 +52,15 @@ async def test_agent_lifecycle():
 
 
 @pytest.mark.asyncio
-async def test_send_unknown_agent(caplog):
+async def test_send_unknown_agent():
     agent = ConcreteTestAgent("sender")
-    msg = InternalMessage(to="unknown_sender", sender="sender", body="boo")
+    msg = InternalMessage(to="unknown_receiver", sender="sender", body="boo")
 
-    with caplog.at_level(logging.WARNING):
-        await agent.send(msg)
+    agent._internal_pub_socket = AsyncMock()
 
-    assert "Attempted to send message to unknown agent: unknown_sender" in caplog.text
+    await agent.send(msg)
+
+    agent._internal_pub_socket.send_multipart.assert_called()
 
 
 @pytest.mark.asyncio