beam: make toposort _key robust to unorderable node types

wizard-work/beam/toposort/toposort.py

@@ -170,7 +170,12 @@ def topo_sort(graph: Mapping[Any, Iterable[Any]]) -> TopoResult:
 
 
 def _key(x):
+    tn = type(x).__name__
+    # Probe whether instances of this type support ordering; if not, fall back
+    # to a repr/id-based key so mixed or unorderable node types never crash
+    # the ready-queue sort or cycle reconstruction.
     try:
-        return (0, type(x).__name__, x)
-    except TypeError:
-        return (1, id(x))
+        _ = x < x
+    except (TypeError, Exception):
+        return (1, tn, repr(x), id(x))
+    return (0, tn, x)