ci: Run unit tests in CI and guard undefined variables

[GuiLeme]

Summary

The unit test suite added by #279 currently runs only when invoked manually. This PR turns it into a pull request regression gate and prevents a pre-existing custom subscription crash exposed by an adversarial variables transition.

The two commits specific to this PR:

1. Add a Unit tests workflow based on the pick-random-user CI pattern, adapted for the SDK: Node.js 20, dependency installation, test:unit:coverage, and coverage artifact upload. It intentionally omits the plugin-specific merge-branches action and removes the dead automated-tests-publish-results.yml workflow, whose expected Automated tests producer does not exist on v0.0.x.
2. Guard the pre-existing variables defined-to-undefined transition before parsing normalized state, with a regression test that verifies unsubscribe and resubscribe behavior. PR 279 reduced this bug's exposure but did not introduce it.

Implements #285

How to verify

npm install
npm run test:unit
npm run test:unit:coverage
npm run build
npm run lint

Locally, all 32 unit tests pass, coverage completes, TypeScript builds successfully, and ESLint reports no errors. The Unit tests workflow runs the same coverage command for pull requests and uploads coverage/ even when the test step fails.