Improve and Add unit tests for untested tool and editor modules

Several core modules in `editor/src/` have no unit test coverage.
Without tests, regressions in these areas can silently break user-facing
functionality and go undetected until manual QA or user bug reports.
This issue tracks the gaps and explains the risk of each untested area.

Start by inventorying the untested modules under editor/src/ and reading the issue's notes about the gaps and risks. Add focused unit tests for the identified modules, then run the relevant test suite; done means the targeted coverage exists and the tests pass.

Written by the indexing model from the issue text.