[jahyunk]

Issue #, if available:

Description of changes:

With LEASE_COUNT_BASED assignment, LeaseCountBasedLeaseAssignmentDecider.balanceWorkerVariance() only moves a lease off a worker that holds more than the target. When every loaded worker holds exactly the target, a new worker gets no leases and stays at 0.

Example from a production fleet: 21520 leases, 1215 active workers, target ceil(21520 / 1215) = 18. 1193 workers hold 18, and 17 workers (including a newly deployed host) hold 0. The leader logs No new lease assignment performed in this iteration on every run, so the new host never processes traffic.

KCL 2.x did not have this gap. DynamoDBLeaseTaker.chooseLeasesToSteal steals 1 lease when the worker needs more than 1 and the most loaded worker has exactly the target:

// steal 1 if we need > 1 and max loaded worker has target leases.
if ((needed > 1) && (numLeasesToSteal == 0)) {
    numLeasesToSteal = 1;
}

This change applies the same rule in the two-pointer loop of balanceWorkerVariance():

• A worker that holds exactly the target (excess == 0) gives 1 lease to a worker that needs more than 1 (needed > 1).
• A worker that needs only 1 lease and has no over-target worker to take from moves the left pointer forward, so the remaining under-target workers are served in the same run.

Unchanged:

• Workers above the target still give first.
• Leases pending a graceful handoff are still skipped.
• When there are at least as many workers as leases (target 1), needed is never above 1, so nothing changes.
• A worker at the target only drops to target - 1, so it does not take a lease back.

Testing:

• Added balanceWorkerVariance_allLoadedWorkersAtTarget_emptyWorkerStealsUpToTargetMinusOne and balanceWorkerVariance_workerNeedsOnlyOneLease_noStealFromWorkersAtTarget.
• mvn -pl amazon-kinesis-client spotless:check passes.

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