test(engine): cover reviewer-consensus-calibration under root vitest (#8349)

The engine package already had a full node:test suite for this module, but
Codecov's codecov/patch gate only reads root vitest coverage — so the
package's largest branch surface (~128 branches) reported ~0% despite real
tests one directory over. Mirror every existing scenario (plus remaining
branch edges) into test/unit/ so the file reaches 100% line/branch coverage
under npm run test:coverage. No source changes.

Author: claytonlin1110