test(FN-1281): expand mission and subtask test coverage
- Extend mission API e2e coverage across CRUD, validation edge cases, observability, interview flows, and autopilot endpoints
- Add regression tests for generated mission/milestone/slice/feature ID handling across route validation paths
- Add subtask-breakdown unit tests for stream manager behavior, JSON parsing/normalization, fallback generation, and error handling
- Cover subtask session lifecycle and SQLite rehydration behavior, including corrupted-row resilience and in-memory fallback precedence
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