Fix scheduler killing healthy running tasks on duplicate dispatch

[dsuhinin]

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When a task instance gets dispatched twice, only one worker actually runs it — the duplicate loses the race and dies. The problem was that the scheduler treated the leftover event from that dead duplicate as a failure and killed the live run that was happily executing.

This fixes both sides of the race:

• Scheduler: before failing a queued task, it now checks whether the task is actually alive — RUNNING with a recent heartbeat. If a worker is clearly still on it, the stale event is ignored. Tasks that genuinely died are still caught later by heartbeat-timeout detection.
• Supervisor: when the server tells a duplicate worker to back off (TaskAlreadyRunningError), it now exits quietly instead of reporting a crash. The duplicate never did any real work, so there's nothing to fail.

possible fix for: #57041

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