Update tests to follow testing-workflow instructions

- Use node:assert instead of assert
- Follow Mock → Run → Assert pattern with clear comments
- Improve test names to describe scenario and expected behavior
- Reorganize imports into logical groups
- Remove "Test X:" prefixes from suite names for cleaner output

Co-authored-by: eleanorjboyd <26030610+eleanorjboyd@users.noreply.github.com>

Author: Copilot