typing: Add type annotations for testscenarios in StringToBytesTest

The StringToBytesTest class uses testscenarios.multiply_scenarios() to dynamically inject test attributes (sign, magnitude, unit_prefix, etc.) at runtime. Mypy cannot infer these dynamically-generated attributes, resulting in 19 “has no attribute” errors during type checking.

This change adds explicit type annotations for the scenarios class attribute and all instance attributes that get injected by the testscenarios framework, allowing mypy to properly validate the test code without errors.