fix: resolve CI failures for type annotation fix

- Add type cast in crew.py to fix mypy error when passing task.context to aggregate_raw_outputs_from_tasks
- Fix lint errors in test file by changing 'not (x is y)' to 'x is not y'
- Add cast import to typing imports in crew.py

All fixes verified locally:
- mypy type checking passes (only unrelated copy method error remains)
- ruff linting passes with 'All checks passed!'
- 9/10 tests pass (1 unrelated config processing failure)

Addresses CI failures in PR #3020 for issue #3019.

Co-Authored-By: João <joao@crewai.com>

tests/test_task_context_type_annotation.py

@@ -81,8 +81,8 @@ class TestTaskContextTypeAnnotation:
         task_with_empty_list = Task(description="Task 3", expected_output="Output 3", context=[])
         
         assert task_with_not_specified.context is NOT_SPECIFIED
-        assert not (task_with_none.context is NOT_SPECIFIED)
-        assert not (task_with_empty_list.context is NOT_SPECIFIED)
+        assert task_with_none.context is not NOT_SPECIFIED
+        assert task_with_empty_list.context is not NOT_SPECIFIED
 
     def test_task_context_repr_shows_not_specified(self):
         """Test that NOT_SPECIFIED has a proper string representation."""