feat: add agent that is able to receive messages from UI

Inside the `/message` enpoint we put a message onto the internal event
queue, which gets read by TestAgent. This agent, in turn, logs the
message (temporary behaviour until we implement RI integration).

The name TestAgent is of course temporary, this is just for exploratory
purposes.

ref: N25B-165

src/control_backend/api/v1/endpoints/message.py

@@ -1,13 +1,23 @@
 from fastapi import APIRouter, Request
 import logging
 
+from zmq import Socket
+
 from control_backend.schemas.message import Message
 
 logger = logging.getLogger(__name__)
 
 router = APIRouter()
 
-# TODO: implement
-@router.post("/message")
+@router.post("/message", status_code=202)
 async def receive_message(message: Message, request: Request):
     logger.info("Received message: %s", message.message)
+
+    topic = b"message"
+    body = message.model_dump_json().encode("utf-8")
+
+    pub_socket: Socket = request.app.state.internal_comm_socket
+
+    pub_socket.send_multipart([topic, body])
+
+    return {"status": "Message received"}