Interrupt dispatch thread in WorkerAction.stop() to unblock blocking work actions

When WorkerAction.stop() is called (e.g. due to task timeout), it signalled
the main execute thread via completed.countDown() but never interrupted the
dispatch thread that is actually running the work action. With process isolation,
the worker JVM cannot exit while that non-daemon dispatch thread is alive, so
DefaultWorkerProcess.waitForStop() blocks for the full duration of any blocking
call inside the work action (e.g. CountDownLatch.await(90s)), causing
TaskTimeoutIntegrationTest to exceed its 60s @IntegrationTestTimeout.

Fix: track the dispatch thread in runningThread (guarded by runningThreadLock).
stop() now sets stopped=true and interrupts runningThread if it is set, causing
any interruptible blocking call in the work action to throw InterruptedException.
run() sets runningThread before executing the action and clears it in a finally
block. It also checks stopped after setting runningThread to handle the race
where stop() ran before run() had a chance to set the field.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>