Type target manager request defaults

[adamtheturtle]

The request-body validator has exactly two default-value sources: CloudDatabase.to_dict() and VuMarkDatabase.to_dict(). Type that parameter as the corresponding CloudDatabaseDict | VuMarkDatabaseDict union instead of an arbitrary Mapping[str, object].

This is independent from #3603 and does not change request parsing or validation behavior.

Local verification:

• all five configured type checkers on target_manager.py
• uv run prek run --all-files
• uv run prek run --all-files --hook-stage pre-push
• uv run pytest tests/mock_vws/test_flask_app_usage.py -k database