fix: harden scan cancellation and cleanup against pre-existing races

[porunov]

Four long-standing issues in the scan-job framework, independent of any particular backend:

• StandardScannerExecutor.interruptTask() dereferenced rowsCollector unconditionally, so a cancel(true) arriving before run() built the collector threw NullPointerException from the future's cancellation path.

• interrupted and rowsCollector crossed the cancelling-thread/scan-thread boundary as plain fields with no happens-before, so a cancel could be lost entirely: interruptTask() might miss the initialized collector while run() missed interrupted=true, letting a cancelled scan run to completion and still invoke finishJob. Both fields are now volatile (whichever way the cancel/setup race goes, one side observes the other) and run() short-circuits after setup when the cancel arrived before the collector existed.

• StandardScannerExecutor.cleanup() set hasCompleted before running the collector cleanup and the transaction rollback; a cleanup failure therefore skipped storeTx.rollback() permanently (subsequent cleanup calls no-op on hasCompleted), leaking the scan's store transaction. Collector cleanup and rollback completion are now tracked separately: the rollback always runs (in a finally block), a rollback failure does not mask the original collector-cleanup failure (attached as suppressed), and a failed rollback stays retryable by the later cleanup attempt instead of being skipped forever.

• Threads.waitForCompletion() swallowed InterruptedException, discarded the caller's interrupt status and kept waiting out the full timeout, so a cancelled scan could not shorten the up-to-3-minute processor wait. It now restores the interrupt flag and returns the current completion state immediately (single caller: StandardScannerExecutor, which logs and proceeds to forced termination).

ScanCancellationTest now also covers cancelling while the scan is still in job setup: cancel(true) must not throw, the future completes as cancelled, the pipeline unwinds, no row is processed and finishJob is never invoked.

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[Copilot]

Pull request overview

This PR hardens scan-job cancellation and cleanup paths to avoid race-triggered NPEs, ensure store transactions are rolled back even when cleanup fails, and make scan cancellation more responsive by honoring thread interrupts during processor shutdown waits.

Changes:

• Preserve interrupt status and stop waiting early in Threads.waitForCompletion(...) when interrupted.
• Prevent interruptTask() from dereferencing a not-yet-initialized rowsCollector during early cancellation.
• Ensure storeTx.rollback() executes even if rowsCollector.cleanup() throws by moving rollback into a finally block.

Reviewed changes

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File	Description
janusgraph-core/src/main/java/org/janusgraph/util/system/Threads.java	Restores interrupt flag and returns early on interruption during completion waits.
janusgraph-core/src/main/java/org/janusgraph/diskstorage/keycolumnvalue/scan/StandardScannerExecutor.java	Hardens cancellation and cleanup to avoid NPE and rollback leaks under races/exceptions.

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