Add unit test coverage for Python UDF operator descriptors (PythonUDFOpDescV2, PythonUDFSourceOpDescV2, PythonTableReducerOpDesc)

[aglinxinyuan]

Task Summary

Three Python-UDF operator descriptors under common/workflow-operator/udf/python/ have no <srcClassName>Spec.scala. Each pins operator metadata, physical-op wiring, schema propagation, and JSON round-trip.

Scope

Source class	Lines	What's currently un-pinned
udf/python/PythonUDFOpDescV2.scala	211	operatorInfo (Python UDF, PYTHON_GROUP, dynamic ports); getPhysicalOp wiring OpExecWithCode(code, "python") with the workers branch + pveName validation; schema propagation (retainInputColumns + outputColumns, collision throws); round-trip
udf/python/source/PythonUDFSourceOpDescV2.scala	124	operatorInfo (1-out Python UDF, PYTHON_GROUP, zero inputs / one output); sourceSchema() from the columns field; round-trip
udf/python/PythonTableReducerOpDesc.scala	67	operatorInfo (Python Table Reducer, PYTHON_GROUP); getOutputSchemas folds the lambdaAttributeUnits into the output schema (throws on the empty default); round-trip

Out of scope

No production-code changes. One <srcClassName>Spec.scala per source class.

Task Type

• Testing / QA