Improve linked list input validation and digit checking

[ANJAN-23]

Description of Change

This PR improves input validation and robustness of the singly linked list implementation.

Key improvements:

• Enhanced digit validation by handling empty strings and using std::isdigit safely with unsigned char.
• Renamed isDigit to is_digit to follow naming conventions.
• Added <cctype> include for proper character classification.
• Improved user input handling in main() to prevent invalid menu and element inputs.
• Ensured the program repeatedly prompts the user until valid numeric input is provided.

These changes make the implementation safer, more user-friendly, and prevent undefined behavior caused by invalid input.

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Checklist

• Added description of change
• Added file name matches file name guidelines
• Added tests and example test must pass
• Added documentation so that the program is self-explanatory and educational
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one
• I acknowledge that all my contributions will be made under the project's license.

Notes: Improves input validation and digit checking for safer user interaction.