Stamp each task with a stable _execution_index at crew-construction time
to lock insertion order as the deterministic tie-break contract.
- Add _execution_index PrivateAttr to Task
- Add stamp_execution_order model_validator to Crew
- Update _execute_tasks and _aexecute_tasks docstrings with ordering contract
- Add comprehensive regression tests
Closes#4664
Co-Authored-By: João <joao@crewai.com>
