Python: get_latest timestamp ties return a checkpoint chosen by save order, can restore stale state

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### Description

`InMemoryCheckpointStorage.get_latest` picks `max()` by timestamp alone. Checkpoints at the same superstep boundary legitimately carry identical timestamps (the `WorkflowCheckpoint` docstring on `iteration_count` says so explicitly), and `datetime.now()` resolution means even distinct boundaries can collide. When timestamps tie, `max()` returns whichever checkpoint the dict iterates first, so the result depends on save order:

```python
storage = InMemoryCheckpointStorage()
parent = WorkflowCheckpoint(workflow_name="w", graph_signature_hash="h", checkpoint_id="parent", timestamp=ts)
child = WorkflowCheckpoint(workflow_name="w", graph_signature_hash="h", checkpoint_id="child", timestamp=ts, previous_checkpoint_id="parent")
# save parent then child -> get_latest returns parent
# save child then parent -> get_latest returns child
```

A workflow resuming via `get_latest` can therefore restore the stale checkpoint, with no error anywhere.

`FileCheckpointStorage.get_latest` has the same timestamp-only `max()`; it happens to mask the issue via list ordering, but the same identical-timestamp input can pick the wrong one.

The checkpoint docstring itself says ordering is defined by the `previous_checkpoint_id` lineage chain, not timestamps alone.

### Expected behavior

Ties on timestamp resolve through the lineage chain: the checkpoint that no other checkpoint supersedes is the latest, regardless of save order.

### Environment

agent-framework python main (astryx-era clone from this week), Python 3.12

Start by locating InMemoryCheckpointStorage.get_latest and FileCheckpointStorage.get_latest, then read the WorkflowCheckpoint docstring, especially iteration_count and previous_checkpoint_id. Verify behavior with identical timestamps saved in both orders; done means the checkpoint not superseded in the lineage chain is selected consistently, regardless of save order.

Written by the indexing model from the issue text.