test: add coverage for GridGenerator.ring and .spiral

[dusan-maintains]

Summary

• Add focused test coverage for GridGenerator.ring() and GridGenerator.spiral().
• Validate distance/radius invariants and uniqueness for ring output.
• Validate center inclusion and expected cell count for spiral output.

Why

• Addresses missing coverage highlighted in issue Test coverage for the Ring and Spiral static methods in GridGenerator.tsx #87.
• Improves confidence around two methods currently marked as potentially untested in source comments.

Test Plan

• npx jest test/src/GridGenerator.test.tsx --runInBand
• Result in this environment: all 9 tests passed.

Notes

• Scope is intentionally limited to tests only; no runtime behavior changes.

[StantonMatt]

Checked this locally on Node v22.22.2 / npm 10.9.7.

The targeted command from the PR passes:

npx jest test/src/GridGenerator.test.tsx --runInBand

Result: 9/9 GridGenerator tests passed, including the new ring and spiral coverage.

I also ran the full suite:

CI=1 npm test -- --runInBand

Result: 7 suites passed, 36 tests passed, 1 skipped, 9 snapshots passed.

The remaining skipped test is Hexagon should render correctly with default props in test/src/Hexagon/Hexagon.test.tsx, so it is separate from the old commented-out GridGenerator ring/spiral placeholder that this PR replaces. No PR checks are currently configured/running, so local verification is the evidence here.