[Testing] Confirm Validation Logic with backend

[dhegberg]

Migration from aws/aws-durable-execution-sdk-python-testing#52

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Context

Verify checkpoint validators accurately model the latest logic in the backend.

This includes payload size checks.

Implementation Details

• File: src/aws_durable_execution_sdk_python_testing/checkpoint/validators

Acceptance Criteria

• Compare existing validators to backend validators and identify new changes that are not in the local runner yet
• Create missing or amend existing validations as appropriate