fix: handle division by zero in calculator

[quantcode-agent]

Summary

• Fixed the division-by-zero bug in src/calculator.ts: the divide function now throws Error("Division by zero is not allowed") when the divisor is zero, instead of silently returning Infinity.
• Added a dedicated test in test/calculator.test.ts that verifies the error is thrown on division by zero.

Assumptions

• Throwing an error is the appropriate behaviour (rather than returning null, NaN, or a result object) since the existing codebase uses simple return values and the test file already uses toThrow style assertions.

Test Results

All 5 tests pass (bun test).