Allow None return annotation on task execute

[snamber]

Summary

• accept Task.execute() -> None when annotations are postponed
• preserve runtime rejection and static type checking for non-None return types
• cover both accepted None and rejected int annotations through the real future-annotations compiler path

Verification

• uv run --package tilebox-workflows pytest tilebox-workflows
• uv run ruff check tilebox-workflows/tilebox/workflows/task.py tilebox-workflows/tests/test_task.py
• uv run ty check tilebox-workflows/tilebox/workflows/task.py tilebox-workflows/tests/test_task.py
• git diff --check

[lukasbindreiter]

aah maybe that's why I never noticed it, does the issue only occur when the __future__.annotations codemod is active?

[lukasbindreiter]

Just verified manually, static analysis tool still flag invalid Task subclasses.