* Refactor expression generation and substitution logic
Updated symbol naming and added safe replacement for expressions.
* Add expr_str to return values in countdown.py
Modified return statement to include the modified expression string.
* Implement test for min_numbers exceeding 10
Add test for CountdownDataset with more than 10 numbers
* Remove trailing-whitespace
* Improve readability of CountdownDataset initialization
Refactor CountdownDataset initialization for readability.