Invalid spec fixtures or expression can contains models other than Expressir::Model::Expressions::XXX?

[kwkwan]

Hi @ronaldtse, some spec fixtures seem invalid.

For example, in the yaml file https://raw.githubusercontent.com/lutaml/expressir/refs/heads/main/spec/syntax/mathematical_functions_schema.yaml (line 11457), the model Expressir::Model::Statements::Return contains expression with _class like Expressir::Model::Literals::Logical.

statements:
  - _class: Expressir::Model::Statements::Return
    expression:
      _class: Expressir::Model::Literals::Logical
      value: :FALSE

It seems that the values of expression are not correct. It is because expression should have _class like Expressir::Model::Expressions::XXX, but not Expressir::Model::Literals::Logical.