test: make killWithEscalation unref regression detectable (#7016)

[atomantic]

Summary

• Assert the escalation timer is unref'd so it cannot keep the Node process alive.
• Closes [test-quality-kill-with-escalation-unref] killWithEscalation unref test only asserts timer is defined, missing event loop leak regression #7016.

Test plan

• ./node_modules/.bin/vitest run lib/killWithEscalation.test.js (8 passed)
• Mutation probe with timer.unref?.() disabled (1 expected failure), then restored
• Four previously timing-sensitive server files with --testTimeout=30000 (346 passed)
• npm test --prefix server (2164 files passed, 4 unrelated 10s timeout failures under the full-suite load)

The full-suite timeout failures are in jsonWritebackConventions.test.js, lib/eidoverseCitySurface.test.js, lib/validationSplit.test.js, and services/sharing/peerSync.test.js; all four pass independently with the higher timeout.