[Testing] Enhance get_assertable_operations method

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

Beginne in src/aws_durable_execution_sdk_python_testing/execution.py bei get_assertable_operations und untersuche anschließend, wie sein Ausführungskontext die abschließende Operation bestimmt. Implementiere die optionale Behandlung von EXECUTION am Ende, einschließlich großer Payloads und der kontextabhängigen Einbeziehung oder Ausschließung, und bewahre dabei den ursprünglichen Ausführungszustand; die Akzeptanzkriterien definieren, wann die Arbeit abgeschlossen ist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.