[python] Introduce 'listagg' aggregator

[PyRSA]

Purpose

Introduce the listagg field aggregator for pypaimon.

This change adds support for the listagg aggregation function in the Python reader, aligning its behavior with the Java implementation. It supports configurable delimiters through fields.<field-name>.list-agg-delimiter and optional deduplication via ields.<field-name>.distinct.

Tests

• python -m pytest pypaimon/tests/test_field_aggregators.py::FieldListaggAggTest -q
• git diff --check

[JingsongLi]

`When configuring an empty delimiter in listagg distinguish, it will directly throw 'ValueError: empty separator'. An empty string is a valid configuration, and all 14 tests have passed, but it does not cover this boundary; The minimum use case has been stably reproduced.

[PyRSA]

`When configuring an empty delimiter in listagg distinguish, it will directly throw 'ValueError: empty separator'. An empty string is a valid configuration, and all 14 tests have passed, but it does not cover this boundary; The minimum use case has been stably reproduced.

Thanks for catching this. Python's str.split("") raises ValueError, while an empty delimiter is a valid configuration. I've updated the implementation to handle this case consistently with the Java behavior and added a test to cover this boundary.

[JingsongLi]

+1