[FLINK-40421][python] Add sorting APIs to DataFrame API

[Milesian111]

What is the purpose of the change

This pull request adds sorting support to the PyFlink DataFrame API. It introduces a lazy DataFrame.sort() transformation backed by the existing Table API order_by() operation, allowing users to sort by one or more columns or expressions without triggering execution.

Brief change log

• Added DataFrame.sort() with support for single-column and multi-column sorting.
• Added scalar and per-column descending options.
• Added scalar and per-column nulls_first options.
• Added validation to reject expressions that already contain .asc or .desc.
• Kept sorting lazy and ensured the original DataFrame remains unchanged.
• Added public API documentation.

Verifying this change

This change added focused tests covering:

• Single-column and multi-column sorting.
• Expression-based sorting.
• Scalar and per-column sort directions.
• Explicit NULL ordering.
• Invalid inputs and mismatched option lengths.
• Lazy plan construction and DataFrame immutability.
• Batch execution with deterministic ordering and NULL handling.

The focused sorting tests and complete DataFrame batch integration tests pass locally. The full DataFrame test module has one unrelated Windows-only failure involving pre-epoch timestamp conversion in time.mktime.

Does this pull request potentially affect one of the following parts:

• Dependencies: no
• The public API: yes
• The serializers: no
• The runtime per-record code paths: no
• Anything that affects deployment or recovery: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? docs

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

• Yes

Generated-by: Codex GPT-5

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CI report:

• f71e1d1 Azure: SUCCESS

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