[Testing] Enhance get_execution_state with paging

[dhegberg]

Migration from aws/aws-durable-execution-sdk-python-testing#34

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Context

Implement proper paging logic in checkpoint processor

Technical Requirements

• Implement proper paging logic in checkpoint processor
• Add next_marker handling for large operation lists
• Add pagination metadata to responses

Implementation Details

• File: src/aws_durable_execution_sdk_python_testing/checkpoint/processor.py:98

Acceptance Criteria

• Proper paging logic implemented in checkpoint processor
• Next_marker handling added for large operation lists
• Pagination metadata added to responses