- Update context field type annotation from Optional[List[Task]] to Union[List[Task], _NotSpecified, None]
- Add ConfigDict(arbitrary_types_allowed=True) to Task model to support _NotSpecified type
- Add comprehensive tests covering the type annotation fix and sentinel behavior
- Fixes issue #3019 where context field default NOT_SPECIFIED didn't match type annotation
The fix maintains backward compatibility while making the type annotation accurate.
The NOT_SPECIFIED sentinel distinguishes between 'not passed' and 'explicitly passed None'.
Co-Authored-By: João <joao@crewai.com>