[FLINK-40335][runtime] Fix Hybrid Shuffle index corruption caused by shared ByteBuffer

[mateczagany]

What is the purpose of the change

ProducerMergedPartitionFileDataIndexRegionHelper holds a reusable ByteBuffer but was exposed as a static singleton shared by every ProducerMergedPartitionFileIndex in the TaskManager.
Each index only guards accesses with its own instance-level lock, so two result partitions spilling or reading index regions concurrently could corrupt in-flight reads and eventually crash the TM.

Brief change log

• Give each index its own helper instance
• Add test

Verifying this change

• Added test that validates concurrent partition file indexes

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

---

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

• Yes (please specify the tool below)

[flinkbot]

CI report:

• 10f2045 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build