[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

Comienza en src/aws_durable_execution_sdk_python_testing/execution.py, en get_assertable_operations, y luego inspecciona cómo su contexto de ejecución determina la operación final. Implementa el manejo opcional del EXECUTION final, incluidos los payloads grandes y la inclusión o exclusión según el contexto, preservando al mismo tiempo el estado de ejecución original; los criterios de aceptación definen cuándo se completa el trabajo.

Escrito por el modelo de indexación a partir del texto del issue.