[FLINK-34335][table] Print query hints in RexSubQuery explain output

[Au-Miner]

What is the purpose of the change

Fix query hints inside RexSubQuery so they are visible in explain output, including sibling subqueries with different join hints.

Brief change log

• Update RelTreeWriterImpl#item to detect RexSubQuery expressions and render their inner rel plans with hint-aware output.
• Share hint rendering through RelTreeWriterImpl#addHintItems, so main explain and subquery explain use the same query hint and query block alias rules.

Verifying this change

Added JoinHintTestBase#testJoinHintWithDifferentHintsInSiblingSubQueries and updated the join hint XML plans for broadcast, shuffle hash, nest loop, and shuffle merge hints to assert subquery hints are printed correctly.

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

[flinkbot]

CI report:

• 31cd29d Azure: FAILURE

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[featzhang]

left a few inline comments, no blocking issues from my side

[featzhang]

left a few inline comments, no blocking issues from my side

[Au-Miner]

Hi @featzhang, thanks for the review. If you have any other questions, please feel free to contact me