fix: handle division-by-zero in calculator

[quantcode-agent]

Summary

• Fixed division-by-zero bug in src/calculator.ts by adding a guard that throws "Cannot divide by zero" when the divisor is 0
• Added test cases in test/calculator.test.ts to verify both normal division and the error case

Changes

• src/calculator.ts: Added if (b === 0) throw new Error("Cannot divide by zero") guard in the divide function
• test/calculator.test.ts: Added test for successful division and test that division by zero throws the expected error

Testing

All 5 tests pass (bun test).

Assumptions

• Throwing an error is the appropriate behaviour for division by zero (rather than returning Infinity or NaN)
• The error message "Cannot divide by zero" follows the project's error messaging conventions