Add calculator_tools example (add, subtract, multiply, divide)

[NA-V10]

Summary

This PR adds a new example module calculator_tools to the examples/ directory.
The design follows the exact structure and style of the existing cipher example,
including licensing headers, CLI behavior, and test format.

Details

• New folder: examples/calculator_tools/
• Exposes simple arithmetic functions: add, subtract, multiply, divide
• Uses Fire without specifying a component (same as cipher.py)
• Proper Google-style docstrings and licensing header
• Includes a full test suite with testutils.BaseTestCase
• Tests cover basic arithmetic operations and division-by-zero error case

Checklist

• Code runs successfully
• Added test file following Google Fire conventions
• Example follows exact layout and style of existing examples
• No changes to core Fire library
• CLA signed