Fix YAML validator and activity file validation errors

- Updated validator terminal step detection to only flag truly terminal steps
- Fixed validator to accept integers and booleans in buckets (as supported by app.py)
- Fixed metadata_remove format in activity17 from dictionary to list of strings
- Added proper terminal section to activity3.yaml without questions/buckets
- Fixed missing restart transition and bucket in activity28
- Removed unused game_end transitions from battleship files
- Updated exit transitions to go directly to step_4 (goodbye step)
- Applied black formatting to validator code

All 30 activity YAML files now validate successfully with 0 errors and 0 warnings.

requirements-test.txt

@@ -3,4 +3,4 @@ pytest-cov
 pytest-mock
 pytest-flask
 pytest-asyncio
-together
+black