[Testing] Enhance get_assertable_operations method

[dhegberg]

Migration from aws/aws-durable-execution-sdk-python-testing#48

---

Context

Add logic to handle optional EXECUTION operation at end of execution

Technical Requirements

• Add logic to handle optional EXECUTION operation at end of execution
• Implement proper detection of final EXECUTION operations with large payloads
• Add context-aware inclusion/exclusion of final EXECUTION operations
• Ensure immutability of original execution state

Implementation Details

• File: src/aws_durable_execution_sdk_python_testing/execution.py:111

Acceptance Criteria

• Logic added to handle optional EXECUTION operation at end of execution
• Proper detection implemented for final EXECUTION operations with large payloads
• Context-aware inclusion/exclusion added for final EXECUTION operations
• Immutability of original execution state ensured