[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

src/aws_durable_execution_sdk_python_testing/execution.py の get_assertable_operations から始め、次に、その実行コンテキストが最終操作をどのように決定するかを調べます。大きなペイロードや、コンテキストに応じた包含または除外を含む、オプションの最終 EXECUTION の処理を実装し、元の実行状態を維持してください。受け入れ基準が、作業完了の条件を定義します。

索引モデルが issue の本文から書いたものです。