refactor: remove proxy_id concept entirely

Not really needed and just adds extra fluff.

test/remote_child_endpoint.zig

@@ -254,7 +254,7 @@ const StdioEndpoint = struct {
                 else
                     return tp.exit_error(error.UnknownWireId, null);
             },
-            .link, .exit, .proxy_id, .transport_error => return tp.exit_error(error.UnexpectedMessage, null),
+            .link, .exit, .transport_error => return tp.exit_error(error.UnexpectedMessage, null),
         }
     }