[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

Commencez dans src/aws_durable_execution_sdk_python_testing/execution.py, au niveau de get_assertable_operations, puis examinez comment son contexte d’exécution détermine l’opération finale. Implémentez le traitement optionnel de EXECUTION final, y compris les grandes charges utiles et l’inclusion ou l’exclusion selon le contexte, tout en préservant l’état d’exécution d’origine ; les critères d’acceptation définissent quand le travail est terminé.

Rédigé par le modèle d'indexation à partir du texte de l'issue.