When fetchMissions returns a non-array (envelope shape, error path, or
mocked detail object) setMissions(data) leaks the bad shape into render,
where missions.filter() throws "is not a function" and crashes the
entire MissionManager test file. Coerce to [] before storing, matching
the existing defenses on the useState init and cache-hydration paths.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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