[unit-testing]: implement NEQ operator and a comprehensive relational expression tests

- Add missing NEQ (!=) operator implementation in expression evaluator
- Create relational_expression_tests module with 40 comprehensive tests
- Cover all relational operators: EQ, NEQ, GT, LT, GTE, LTE
- Test integer, real, and mixed type comparisons
- Include edge cases: negatives, zero, decimals, large numbers
- Add error handling tests for invalid types and undefined variables
- Test complex expressions with variables and arithmetic operations
- Maintain proper test organization in expression_eval.rs

Author: rbonifacio