[Testing] Enhance get_assertable_operations method

Migration from https://github.com/aws/aws-durable-execution-sdk-python-testing/issues/48

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## 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

从 src/aws_durable_execution_sdk_python_testing/execution.py 中的 get_assertable_operations 开始，然后检查其执行上下文如何确定最终操作。实现可选的最终 EXECUTION 处理，包括大型 payload 以及根据上下文进行包含或排除，同时保留原始执行状态；验收标准定义了何时完成工作。

由索引模型根据 Issue 内容生成。