Python: checkpoints flatten dict subclasses (defaultdict/Counter/OrderedDict become plain dict) #8517

Motivation & Context

workflow checkpoint state that contains dict subclasses (defaultdict, Counter, OrderedDict) loses its type through a save/load round-trip: it comes back as a plain dict. the encode path uses isinstance(value, dict) and reconstructs a plain dict on decode, even though the module documents full python object fidelity and the unpickler allowlist already covers collections types.

workflows that resume from a checkpoint crash or silently change behavior when their state was a defaultdict or Counter (missing-key defaulting, counter arithmetic).

Reproduction

from collections import Counter
# executor state: {"counts": Counter({"a": 2})}
# save via FileCheckpointStorage, then load:
type(state["counts"])  # dict, not Counter

new tests in the PR build the round-trip and assert the subclass survives.

Expected behavior

dict subclasses round-trip with their type intact.

env: python main