[Testing] Confirm Validation Logic with backend #452

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

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