[FLINK-39531][table-planner] Fix duplicated Python UDF extraction in calc split rule

[auroflow]

What is the purpose of the change

Currently, ScalarFunctionSplitter bookkeeps extracted RexNodes to deduplicate identical remote functions during calc splitting, but the bookkeeping is keyed on the RexInputRef rather than the original RexNode. Therefore, when the same Python or Async UDF call appears in the input of multiple projections of one SELECT, each occurrence is extracted once and the UDF is invoked every time.

This PR solves this problem by making the bookkeeping keyed on the original RexNodes, not the RexInputRef.

Brief change log

• Make the bookkeeping in ScalarFunctionSplitter keyed on the original RexNodes, not the RexInputRef.

Verifying this change

This change added tests and can be verified as follows:

• Added a test on duplicated Python async functions

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)

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CI report:

• 036f1a2 Azure: FAILURE

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