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>

src/crewai/crew.py

@@ -1034,7 +1034,7 @@ class Crew(FlowTrackable, BaseModel):
         context = (
             aggregate_raw_outputs_from_task_outputs(task_outputs)
             if task.context is NOT_SPECIFIED
-            else aggregate_raw_outputs_from_tasks(task.context)
+            else aggregate_raw_outputs_from_tasks(cast(List["Task"], task.context))
         )
         return context