[FLINK-40344][table] Support selecting computed grouping expressions

[beetle0915]

What is the purpose of the change

This change fixes the selection of computed grouping expressions in the Table API.

Previously, a computed grouping expression such as a > 10 was materialized as an aggregate output field, but a subsequent projection attempted to resolve the original expression against the aggregate output. Since the original field a was no longer available, validation failed.

Brief change log

• Assign stable output names to computed grouping expressions.
• Replace matching grouping expressions in aggregate projections with references to the corresponding aggregate output fields.
• Unwrap grouping expression aliases when serializing GROUP BY.
• Add regression tests for direct reuse and structurally equivalent computed grouping expressions.
• Cover both groupBy(...).select(...) and explicit groupBy(...).aggregate(...).select(...) paths while ensuring that non-grouping expressions remain rejected.

Verifying this change

This change added tests in QueryOperationSqlSerializationTest covering:

• Selecting the same computed grouping expression.
• Selecting a structurally equivalent computed grouping expression with an alias.
• Selecting a computed grouping expression after an explicit aggregate() call.
• Ensuring that a non-grouping expression is still rejected.

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

• Yes

Generated-by: OpenAI Codex (GPT-5)

[flinkbot]

CI report:

• cd82e7c Azure: SUCCESS

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[beetle0915]

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[beetle0915]

The latest Azure CI run has passed successfully on the current head (cd82e7c). This PR is ready for review. Thanks!