Fix restart aggregation when remote partitions are already completed

[answndud]

Summary

• Return completed partition executions from SimpleStepExecutionSplitter on restart so the manager can aggregate them.
• Update AbstractPartitionHandler to skip re-executing already completed partitions while still returning them in the final results.
• Add coverage in splitter/handler/partition-step tests for restart aggregation with completed partitions.

Testing

• mvn -pl spring-batch-core -Dtest=TaskExecutorPartitionHandlerTests,SimpleStepExecutionSplitterTests,PartitionStepTests test
• In this local environment, compilation fails before tests due NullAway JSpecify requirements (JDK 22+ or -XDaddTypeAnnotationsToSymbol=true).

Closes #5158