[FLINK-39034][streaming] Fix timestamp loss in SortPartitionOperator

[Jackeyzhe]

What is the purpose of the change

This pull request fixes the timestamp loss issue in SortPartitionOperator when running in STREAMING mode.

When users use fullWindowPartition().sortPartition() operator combination in Flink DataStream API, the timestamp was being lost during SortPartitionOperator processing, causing ctx.timestamp() in downstream KeyedProcessFunction to return null.

Brief change log

Modified SortPartitionOperator.java, add timestamp in sorter and set in streamrecord when collecting output

Verifying this change

fix unit test in SortPartitionOperatorTest.java

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

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CI report:

• 448fc32 Azure: FAILURE

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