fix(evaluator): distinguish interrupted and failed sigterm exits

[ngoncharenko]

Summary

• add graceful SIGTERM interruption markers and configurable shutdown timeout handling in nemo-evaluator
• distinguish adapter server fatal shutdowns from external interruptions so server-triggered SIGTERM exits nonzero instead of looking successful
• update local, SLURM, and Lepton launcher flows to treat interrupted runs as non-success and skip auto-export
• add unit coverage for shutdown handling and launcher classification paths

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[wprazuch]

Hey @ngoncharenko !
The code looks good to me, but I would like to give it a try with some real-world use cases before approving. ETA for this is Tuesday next week (Monday is bank holiday in Poland). Sorry for waiting, and once again thanks for the contribution! :)