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

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### 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

```python
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

Start at FileCheckpointStorage and trace the checkpoint encode/decode path, then review the existing unpickler allowlist for collections types. Run the checkpoint round-trip tests and add coverage showing defaultdict, Counter, and OrderedDict retain their types after save and load.

Written by the indexing model from the issue text.