test: allow parallel branch observation ordering

[zhongkechen]

Summary

• make the parallel named branches example test assert the full set of child branch names
• stop depending on child operation list order for parallel branches

Why this is needed

The first attempt of PR #512 failed in the Python 3.12 CI job because test_parallel_with_named_branches expected the first child operation to be fetch-user-data, but the observed first child was fetch-orders.

The example runs branches with ParallelConfig max_concurrency set to 3, so child operations can be recorded or surfaced in execution/checkpoint observation order rather than declaration order. The test only needs to verify that named branches and default branch names are present, not that parallel child operations appear in a stable order.

Verification

• hatch run dev-examples:test packages/aws-durable-execution-sdk-python-examples/test/parallel/test_parallel_with_named_branches.py
• hatch fmt --check

issue: #405