test(integration): cover lifecycle error paths

[vernonstinebaker]

Summary

• add assembled-server integration coverage for lifecycle error responses when an instance is missing or its binary cannot be started
• verify start and stop return 404 for a missing managed instance
• verify start and restart return 500 for a seeded managed instance whose binary is absent

Validation

• zig build test-integration -Dembed-ui=false -Dbuild-ui=false --summary all
• zig build test -Dembed-ui=false -Dbuild-ui=false --summary all