Two engine test files fail on current `main` (17c4007). Both are stale
**test-mock** breakages — no production code is touched.
### 1. `restart.integration.test.ts`
Its `vi.mock("../pi.js", …)` factory replaces the module wholesale but
omits `ModelFallbackExhaustedError`. `triage.ts` guards its catch block
with `err instanceof ModelFallbackExhaustedError` (imported from
`pi.js`), so evaluating that guard throws *"No
ModelFallbackExhaustedError export is defined on the mock"*.
Fix: export a plain `Error`-subclass stub from the factory. No restart
test enters the fallback-exhausted branch, so `instanceof` simply
returns `false` — a faithful stub.
### 2. `reliability-interactions/mission-validation-trigger-gap.test.ts`
Two recovery-path `missionStore` mocks omit `getMission`. #1910's
mission-active gate now walks `getSlice → getMilestone → getMission`
inside `resolveFeatureMission`. The resulting throw is swallowed by
`processTaskOutcome`'s `catch`, aborting recovery before it can ensure
assertions / start the validator run — surfacing as
`ensureFeatureAssertionLinked` asserted called-once but seen 0 times.
Fix: add `getMission` returning an active mission to both mocks.
### Verification
```
npx vitest run src/__tests__/reliability-interactions/mission-validation-trigger-gap.test.ts src/__tests__/restart.integration.test.ts
Test Files 2 passed (2)
Tests 54 passed (54)
```
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **Tests**
* Improved coverage for mission recovery and restart flows, making
validation scenarios more reliable.
* Fixed test mocks so recovery and triage paths can run without
unexpected errors during assertions.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
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