Use optional for nullable input object resolver arguments

[Copilot]

Nullable input objects could produce mismatched resolver signatures and conversions, particularly for fields returning built-in scalars. Generated code declared std::optional<T> but passed std::unique_ptr<T>, causing compilation failures.

• Argument classification

  • Classify field arguments before handling built-in return types.

• Type generation

  • Use std::optional<T> consistently for nullable input-object resolver arguments.
  • Preserve std::unique_ptr<T> for recursive input-object members.

getField0(FieldParams&&, std::optional<Type>&&);
getField1(FieldParams&&, std::optional<Type>&&);

• Regression coverage
  • Cover nullable input objects on both scalar- and object-returning fields.

• Fixes Error compiling in resolve function #354