test(gjc): stabilize worker guard timeout coverage

[devswha]

Summary

• keep the production guard timer unref behavior unchanged
• add a test-only referenced timer while the timeout rejection is asserted
• allow the isolated worker supervisor test process to reach all remaining cases

Verification

• isolated worker client tests: 19/19, 0 cancelled
• full npm test: 0 cancelled (the three known live-session fixture failures are addressed separately in test(gjc): isolate live session synchronizer fixtures #1)
• npm run typecheck
• npm run lint
• npm run build