fix: make all tests pass (calculator, string-utils, task-manager, date-utils, validator)

[stooit]

Summary

Fixes all 16 previously-failing tests across the utility library. The full suite now passes: 60 passing / 0 failing. No test files were modified and no dependencies were added.

Changes

• src/calculator.ts — divide now throws Error("Division by zero") when the divisor is 0.
• src/string-utils.ts — wordCount now trims and splits on \s+ so runs of whitespace don't create empty tokens; truncate implemented to append whole words up to the limit.
• src/task-manager.ts — completed missing/incomplete TaskManager methods to match expected behaviour.
• src/date-utils.ts — fixed an off-by-one in relative-time formatting (use Math.floor rather than Math.round).
• src/validator.ts — isUrl now accepts URLs with an explicit port (e.g. http://localhost:3000).

Verification

bun test → 60 pass, 0 fail, 70 expect() calls across 5 files.

Assumptions

• Behaviour was reverse-engineered from the existing tests, which were treated as the source of truth and left unmodified.
• Division-by-zero error message text matches what the calculator test asserts.

🤖 Generated autonomously.