Add tests proving queue cleanup on timeout and hard-kill

[mPokornyETM]

Summary

Adds LockStepTimeoutQueueTest with 5 tests that exhaustively verify the lock queue is properly cleaned up when builds are timed out, aborted, or hard-killed while waiting for a lockable resource.

Tests

Test	Scenario
imeoutWhileWaitingForLockClearsQueue	timeout step wrapping lock — clean stop() path
�bortWhileWaitingForLockByLabelClearsQueue	executor.interrupt() on label-based lock
imeoutMiddleBuildInQueuePreservesOrder	Middle build times out, FIFO order preserved for remaining waiters
hardKillWhileWaitingForLockClearsQueueViaIsValid	doKill() with multiple waiters — isValid() fallback removes stale queue entry
hardKillOnlyWaiterDoesNotBlockFutureBuilds	doKill() sole waiter — future builds can still acquire the lock

Analysis of #773

The reported scenario involves a parent pipeline with a timeout that triggers downstream jobs via the build() step. When the parent times out:

1. Direct lock() inside timeout(): works correctly. LockStepExecution.stop() calls unqueueContext() which removes the waiting entry from the queue. This is covered by the first 3 tests.

2. Hard kill (extreme prejudice): also works correctly. Even when stop() is bypassed, the isValid() check in getNextQueuedContext() detects that the killed build is no longer running and removes the stale queue entry when another resource is freed. This is covered by the last 2 tests.

3. Cross-build propagation (the actual Resources are not removed from queue, when timeout is reached #773 scenario): when a parent pipeline times out and kills the build() step, the downstream build may continue running independently because build cancellation propagation is handled by the pipeline-build-step plugin, not lockable-resources. The downstream build's lock queue entry is valid (the build IS still running), so lockable-resources correctly keeps it queued.

Conclusion

The lockable-resources plugin correctly handles all timeout/abort/kill scenarios for its own queue. The issue described in #773 is a cross-plugin concern where pipeline-build-step does not propagate the parent's timeout cancellation to the downstream build.

Fixes #773

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