Add Nullable<T> support to command parameter parser selection - #575

The command parser factory did not recognize nullable value-type parameters (e.g. int?, float?), which blocked optional nullable command arguments. This change makes parser resolution nullable-aware by reusing existing parsers for the underlying type.

• Parser factory: nullable-aware type resolution

  • Updated DefaultCommandParameterParserFactory to unwrap Nullable<T> before parser lookup.
  • Existing parser mappings now apply to nullable variants without introducing new parser implementations:
    • int? → IntParser
    • float? → FloatParser
    • double? → DoubleParser
    • bool? → BooleanParser
    • Enum? → EnumParser

• Test coverage: nullable mapping cases

  • Extended DefaultCommandParameterParserFactoryTests with focused cases for:
    • int?, float?, double?, bool?
    • nullable enum (DayOfWeek?)
  • Ensures nullable mappings stay aligned with non-nullable parser behavior.

• Integration-style execution coverage

  • Added PlayerCommandServiceTests cases to validate end-to-end behavior from input string through dispatch and execution for:
    • /money 123 → method receives int? value 123
    • /money → method receives default null
  • Confirms returning a boxed int from parsing works correctly when invoking command methods with nullable parameters.

Example usage now supported:

[PlayerCommand]
public void MoneyCommand(Player player, int? money = null) { }