Guard terminal runs against late heartbeats

[owenshuo]

Summary

• add a run heartbeat monitor with durable terminal states for completed, failed, and cancelled runs
• reject late worker heartbeats once a run reaches a terminal outcome so the run cannot be revived as running
• wire the monitor into task execution and expose regression coverage for terminal idempotency and engine-level late heartbeat handling

Fixes #3913

Validation

• python3 -m pytest tests/test_heartbeat_monitor.py tests/test_scheduler.py tests/test_config.py -q
• python3 -m flake8 src/orchestrator/engine.py src/orchestrator/init.py tests/test_heartbeat_monitor.py
• git diff --check