chore: adjust message/command structure and write unit tests

ref: N25B-205

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

@@ -0,0 +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()
+
+@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"}