[FLINK-39034][runtime] Preserve timestamps in SortPartitionOperator

[Jackeyzhe]

What is the purpose of the change

SortPartitionOperator currently writes only record values into the external sorter for non-key-selector sorting. When records are emitted after sorting, this recreates output records without preserving the original StreamRecord timestamp state.

This change stores StreamRecord instances in the sorter and compares their wrapped values through a StreamRecordComparator, so sorted output preserves whether the input record had a timestamp and, when present, the original timestamp.

Brief change log

• Store StreamRecord<INPUT> in the non-key-selector sorter path.
• Add StreamRecordComparator to compare records by their wrapped values while preserving stream record metadata.
• Extend SortPartitionOperatorTest to verify sorted output keeps timestamps and no-timestamp records remain timestamp-less.

Verifying this change

This change is covered by existing and extended tests.

Verified locally with:

./mvnw -pl flink-runtime -am -Dtest=SortPartitionOperatorTest -Dsurefire.failIfNoSpecifiedTests=false test

The target test passed with 2 tests, 0 failures, 0 errors, and 0 skipped.

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: yes
• The runtime per-record code paths (performance sensitive): yes
• 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

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Generated-by: OpenAI GPT-5.5, reviewd by human
I'm responsible for the quality and correctness of every change in this PR regardless of the tooling used.

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CI report:

• fad499c Azure: SUCCESS

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