fix: handle uncaught UnicodeDecodeError

UnicodeDecodeError occurred when trying to connect to the server with HTTPS. The error
was not caught by the broad Exception clause in `handle_agent`, causing a traceback
to be dumped.

server/src/main.py

@@ -87,6 +87,8 @@ class Server:
             conn.send(json.dumps({"command": command}).encode())
         except json.decoder.JSONDecodeError:
             logger.error(f"Invalid JSON received from {addr}")
+        except UnicodeDecodeError:
+            logger.error(f"Invalid UTF-8 received from {addr}")
         except Exception as e:
             logger.error(f"Error handling agent {agent_id}: {e}")
         finally: